Gray's Anatomy for Students PDF

GRAY’S ANATOMY FOR STUDENTS Fourth Edition Richard L. Drake, PhD, FAAA Director of Anatomy Professor of Surgery Cleveland Clinic Lerner College of Medicine Case Western Reserve University Cleveland, Ohio A. Wayne Vogl, PhD, FAAA Professor of A...

GRAY’S ANATOMY FOR STUDENTS Fourth Edition Richard L. Drake, PhD, FAAA Director of Anatomy Professor of Surgery Cleveland Clinic Lerner College of Medicine Case Western Reserve University Cleveland, Ohio A. Wayne Vogl, PhD, FAAA Professor of Anatomy and Cell Biology Department of Cellular and Physiological Sciences Faculty of Medicine University of British Columbia Vancouver, British Columbia, Canada Adam W. M. Mitchell, MB BS, FRCS, FRCR Consultant Radiologist Director of Radiology Fortius Clinic London, United Kingdom Illustrations by Richard Tibbitts and Paul Richardson Photographs by Ansell Horn GRAY’S ANATOMY FOR STUDENTS, FOURTH EDITION ISBN: 978-0-323-39304-1 IE ISBN: 978-0-323-61104-6 Copyright © 2020 Elsevier Inc. All rights reserved. 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Typically, when used by This includes the vasculature, the nerves, the bones, itself, the term anatomy tends to mean gross or macroscopic the muscles, and all other structures and organs anatomy—that is, the study of structures that can be seen located in the region of the body defined as the without using a microscopic. Microscopic anatomy, also thorax. After studying this region, the other regions of called histology, is the study of cells and tissues using a the body (i.e., the abdomen, pelvis, lower limb, upper microscope. limb, back, head, and neck) are studied in a similar Anatomy forms the basis for the practice of medicine. fashion. Anatomy leads the physician toward an understanding of In contrast, in a systemic approach, each system of a patient’s disease, whether he or she is carrying out a the body is studied and followed throughout the entire physical examination or using the most advanced imaging body. For example, a study of the cardiovascular system techniques. Anatomy is also important for dentists, chiro- looks at the heart and all of the blood vessels in the body. practors, physical therapists, and all others involved in any When this is completed, the nervous system (brain, aspect of patient treatment that begins with an analysis of spinal cord, and all the nerves) might be examined in clinical signs. The ability to interpret a clinical observation detail. This approach continues for the whole body until correctly is therefore the endpoint of a sound anatomical every system, including the nervous, skeletal, muscular, understanding. gastrointestinal, respiratory, lymphatic, and reproduc- Observation and visualization are the primary tech- tive systems, has been studied. niques a student should use to learn anatomy. Anatomy is much more than just memorization of lists of names. Each of these approaches has benefits and deficiencies. Although the language of anatomy is important, the The regional approach works very well if the anatomy network of information needed to visualize the position of course involves cadaver dissection but falls short when physical structures in a patient goes far beyond simple it comes to understanding the continuity of an entire memorization. Knowing the names of the various branches system throughout the body. Similarly, the systemic of the external carotid artery is not the same as being able approach fosters an understanding of an entire system to visualize the course of the lingual artery from its origin throughout the body, but it is very difficult to coordinate in the neck to its termination in the tongue. Similarly, this directly with a cadaver dissection or to acquire suffi- understanding the organization of the soft palate, how it is cient detail. related to the oral and nasal cavities, and how it moves during swallowing is very different from being able to recite the names of its individual muscles and nerves. An under- Important anatomical terms standing of anatomy requires an understanding of the The anatomical position context in which the terminology can be remembered. The anatomical position is the standard reference position of the body used to describe the location of structures (Fig. 1.1). The body is in the anatomical position when standing How can gross anatomy be studied? upright with feet together, hands by the side and face The term anatomy is derived from the Greek word temnein, looking forward. The mouth is closed and the facial expres- meaning “to cut.” Clearly, therefore, the study of anatomy sion is neutral. The rim of bone under the eyes is in the is linked, at its root, to dissection, although dissection of same horizontal plane as the top of the opening to the cadavers by students is now augmented, or even in some ear, and the eyes are open and focused on something in cases replaced, by viewing prosected (previously dissected) the distance. The palms of the hands face forward with the material and plastic models, or using computer teaching fingers straight and together and with the pad of the thumb modules and other learning aids. turned 90° to the pads of the fingers. The toes point Anatomy can be studied following either a regional or a forward. systemic approach. Anatomical planes With a regional approach, each region of the body Three major groups of planes pass through the body in the 2 is studied separately and all aspects of that region anatomical position (Fig. 1.1).  What Is Anatomy Important Anatomical Terms 1 Superior Coronal plane Inferior margin of orbit level with top of external auditory meatus Face looking forward Sagittal plane Anterior Posterior Medial Transverse, horizontal, or axial plane Hands by sides palms forward Lateral Feet together toes forward Inferior Fig. 1.1 The anatomical position, planes, and terms of location and orientation. 3  The Body Coronal planes are oriented vertically and divide the Proximal and distal are used with reference to being body into anterior and posterior parts. closer to or farther from a structure’s origin, particu- Sagittal planes also are oriented vertically but are at larly in the limbs. For example, the hand is distal to the right angles to the coronal planes and divide the body elbow joint. The glenohumeral joint is proximal to into right and left parts. The plane that passes through the elbow joint. These terms are also used to describe the center of the body dividing it into equal right and the relative positions of branches along the course of left halves is termed the median sagittal plane. linear structures, such as airways, vessels, and nerves. Transverse, horizontal, or axial planes divide the For example, distal branches occur farther away toward body into superior and inferior parts. the ends of the system, whereas proximal branches occur closer to and toward the origin of the system. Terms to describe location Cranial (toward the head) and caudal (toward the tail) Anterior (ventral) and posterior (dorsal), are sometimes used instead of superior and inferior, medial and lateral, superior and inferior respectively. Three major pairs of terms are used to describe the location Rostral is used, particularly in the head, to describe the of structures relative to the body as a whole or to other position of a structure with reference to the nose. For structures (Fig. 1.1). example, the forebrain is rostral to the hindbrain. Anterior (or ventral) and posterior (or dorsal) Superficial and deep describe the position of structures relative to the “front” Two other terms used to describe the position of structures and “back” of the body. For example, the nose is an in the body are superficial and deep. These terms are anterior (ventral) structure, whereas the vertebral used to describe the relative positions of two structures column is a posterior (dorsal) structure. Also, the nose with respect to the surface of the body. For example, the is anterior to the ears and the vertebral column is pos- sternum is superficial to the heart, and the stomach is deep terior to the sternum. to the abdominal wall. Medial and lateral describe the position of structures Superficial and deep can also be used in a more absolute relative to the median sagittal plane and the sides of fashion to define two major regions of the body. The super- the body. For example, the thumb is lateral to the little ficial region of the body is external to the outer layer of finger. The nose is in the median sagittal plane and deep fascia. Deep structures are enclosed by this layer. is medial to the eyes, which are in turn medial to the Structures in the superficial region of the body include the external ears. skin, superficial fascia, and mammary glands. Deep struc- Superior and inferior describe structures in reference tures include most skeletal muscles and viscera. Superficial to the vertical axis of the body. For example, the head is wounds are external to the outer layer of deep fascia, superior to the shoulders and the knee joint is inferior whereas deep wounds penetrate through it. to the hip joint. Proximal and distal, cranial and caudal, and rostral Other terms used to describe positions include proximal and distal, cranial and caudal, and rostral. 4  Imaging Diagnostic Imaging Techniques 1 Imaging Diagnostic imaging techniques Tungsten filament Tungsten target In 1895 Wilhelm Roentgen used the X-rays from a cathode Focusing cup Glass X-ray tube ray tube to expose a photographic plate and produce the first radiographic exposure of his wife’s hand. Over the past 35 years there has been a revolution in body imaging, which has been paralleled by developments in computer technology. Plain radiography X-rays are photons (a type of electromagnetic radiation) and are generated from a complex X-ray tube, which is a type of cathode ray tube (Fig. 1.2). The X-rays are then Cathode Anode X-rays collimated (i.e., directed through lead-lined shutters to stop them from fanning out) to the appropriate area of the body. As the X-rays pass through the body they are attenuated (reduced in energy) by the tissues. Those X-rays that pass Fig. 1.2 Cathode ray tube for the production of X-rays. through the tissues interact with the photographic film. In the body: air attenuates X-rays a little; fat attenuates X-rays more than air but less than water; and bone attenuates X-rays the most. These differences in attenuation result in differences in the level of exposure of the film. When the photographic film is developed, bone appears white on the film because this region of the film has been exposed to the least amount of X-rays. Air appears dark on the film because these regions were exposed to the greatest number of X-rays. Modifications to this X-ray technique allow a continu- ous stream of X-rays to be produced from the X-ray tube and collected on an input screen to allow real-time visual- ization of moving anatomical structures, barium studies, Fig. 1.3 Fluoroscopy unit. angiography, and fluoroscopy (Fig. 1.3). 5  The Body injections, so the necessary precautions must be taken. Contrast agents Intra-arterial and intravenous contrast agents not only To demonstrate specific structures, such as bowel loops or help in visualizing the arteries and veins but because they arteries, it may be necessary to fill these structures with a are excreted by the urinary system, can also be used to substance that attenuates X-rays more than bowel loops or visualize the kidneys, ureter, and bladder in a process arteries do normally. It is, however, extremely important known as intravenous urography. that these substances are nontoxic. Barium sulfate, an insoluble salt, is a nontoxic, relatively high-density agent Subtraction angiography that is extremely useful in the examination of the gastro- During angiography it is often difficult to appreciate the intestinal tract. When a barium sulfate suspension is contrast agent in the vessels through the overlying bony ingested it attenuates X-rays and can therefore be used to structures. To circumvent this, the technique of subtrac- demonstrate the bowel lumen (Fig. 1.4). It is common to tion angiography has been developed. Simply, one or add air to the barium sulfate suspension, by either ingest- two images are obtained before the injection of contrast ing “fizzy” granules or directly instilling air into the body media. These images are inverted (such that a negative is cavity, as in a barium enema. This is known as a double- created from the positive image). After injection of the contrast (air/barium) study. contrast media into the vessels, a further series of images For some patients it is necessary to inject contrast agents are obtained, demonstrating the passage of the contrast directly into arteries or veins. In this case, iodine-based through the arteries into the veins and around the circula- molecules are suitable contrast agents. Iodine is chosen tion. By adding the “negative precontrast image” to the because it has a relatively high atomic mass and so mark- positive postcontrast images, the bones and soft tissues edly attenuates X-rays, but also, importantly, it is naturally are subtracted to produce a solitary image of contrast excreted via the urinary system. Intra-arterial and intrave- only. Before the advent of digital imaging this was a nous contrast agents are extremely safe and are well toler- challenge, but now the use of computers has made this ated by most patients. Rarely, some patients have an technique relatively straightforward and instantaneous anaphylactic reaction to intra-arterial or intravenous (Fig. 1.5). Fig. 1.4 Barium sulfate follow-through. Fig. 1.5 Digital subtraction angiogram. 6  Imaging Diagnostic Imaging Techniques 1 Ultrasound Doppler ultrasound Ultrasonography of the body is widely used for all aspects Doppler ultrasound enables determination of flow, its of medicine. direction, and its velocity within a vessel using simple Ultrasound is a very high frequency sound wave ultrasound techniques. Sound waves bounce off moving (not electromagnetic radiation) generated by piezoelectric structures and are returned. The degree of frequency shift materials, such that a series of sound waves is produced. determines whether the object is moving away from or Importantly, the piezoelectric material can also receive the toward the probe and the speed at which it is traveling. sound waves that bounce back from the internal organs. Precise measurements of blood flow and blood velocity can The sound waves are then interpreted by a powerful therefore be obtained, which in turn can indicate sites of computer, and a real-time image is produced on the blockage in blood vessels. display panel. Developments in ultrasound technology, including the Computed tomography size of the probes and the frequency range, mean that a Computed tomography (CT) was invented in the 1970s by broad range of areas can now be scanned. Sir Godfrey Hounsfield, who was awarded the Nobel Prize Traditionally ultrasound is used for assessing the in Medicine in 1979. Since this inspired invention there abdomen (Fig. 1.6) and the fetus in pregnant women. have been many generations of CT scanners. Ultrasound is also widely used to assess the eyes, neck, soft A CT scanner obtains a series of images of the body tissues, and peripheral musculoskeletal system. Probes (slices) in the axial plane. The patient lies on a bed, an have been placed on endoscopes, and endoluminal ultra- X-ray tube passes around the body (Fig. 1.7), and a series sound of the esophagus, stomach, and duodenum is now of images are obtained. A computer carries out a complex routine. Endocavity ultrasound is carried out most com- mathematical transformation on the multitude of images monly to assess the genital tract in women using a to produce the final image (Fig. 1.8). transvaginal or transrectal route. In men, transrectal ultrasound is the imaging method of choice to assess the Magnetic resonance imaging prostate in those with suspected prostate hypertrophy or Nuclear magnetic resonance imaging was first described in malignancy. 1946 and used to determine the structure of complex Fig. 1.6 Ultrasound examination of the abdomen. Fig. 1.7 Computed tomography scanner. 7  The Body molecules. The process of magnetic resonance imaging (MRI) is dependent on the free protons in the hydrogen nuclei in molecules of water (H2O). Because water is present in almost all biological tissues, the hydrogen proton is ideal. The protons within a patient’s hydrogen nuclei can be regarded as small bar magnets, which are randomly oriented in space. The patient is placed in a strong magnetic field, which aligns the bar magnets. When a pulse of radio waves is passed through the patient the magnets are deflected, and as they return to their aligned position they emit small radio pulses. The strength and frequency of the emitted pulses and the time it takes for the protons to return to their pre-excited state produce a signal. These signals are analyzed by a powerful computer, and an image is created (Fig. 1.9). By altering the sequence of pulses to which the protons are subjected, different properties of the protons can be Fig. 1.8 Computed tomography scan of the abdomen at vertebral level L2. assessed. These properties are referred to as the “weight- ing” of the scan. By altering the pulse sequence and the scanning parameters, T1-weighted images (Fig. 1.10A) and T2-weighted images (Fig. 1.10B) can be obtained. These two types of imaging sequences provide differences in image contrast, which accentuate and optimize different tissue characteristics. From the clinical point of view: Most T1-weighted images show dark fluid and bright fat—for example, within the brain the cerebrospinal fluid (CSF) is dark. T2-weighted images demonstrate a bright signal from fluid and an intermediate signal from fat—for example, in the brain the CSF appears white. MRI can also be used to assess flow within vessels and to produce complex angiograms of the peripheral and cerebral circulation. Diffusion-weighted imaging Fig. 1.9 A T2-weighted MR image in the sagittal plane of the Diffusion-weighted imaging provides information on the pelvic viscera in a woman. degree of Brownian motion of water molecules in various tissues. There is relatively free diffusion in extracellular spaces and more restricted diffusion in intracellular The important difference between gamma rays and spaces. In tumors and infarcted tissue, there is an increase X-rays is that gamma rays are produced from within the in intracellular fluid water molecules compared with nucleus of an atom when an unstable nucleus decays, the extracellular fluid environment resulting in overall whereas X-rays are produced by bombarding an atom with increased restricted diffusion, and therefore identification electrons. of abnormal from normal tissue. For an area to be visualized, the patient must receive a gamma ray emitter, which must have a number of proper- ties to be useful, including: Nuclear medicine imaging Nuclear medicine involves imaging using gamma rays, a reasonable half-life (e.g., 6 to 24 hours), 8 which are another type of electromagnetic radiation. an easily measurable gamma ray, and  Imaging Nuclear Medicine Imaging 1 energy deposition in as low a dose as possible in the patient’s tissues. The most commonly used radionuclide (radioisotope) is technetium-99m. This may be injected as a technetium salt or combined with other complex molecules. For example, by combining technetium-99m with methylene diphosphonate (MDP), a radiopharmaceutical is produced. When injected into the body this radiopharmaceutical specifically binds to bone, allowing assessment of the skeleton. Similarly, combining technetium-99m with other compounds permits assessment of other parts of the body, for example the urinary tract and cerebral blood flow. Depending on how the radiopharmaceutical is absorbed, distributed, metabolized, and excreted by the body after injection, images are obtained using a gamma camera (Fig. 1.11). Positron emission tomography Positron emission tomography (PET) is an imaging modality for detecting positron-emitting radionuclides. A A positron is an anti-electron, which is a positively charged particle of antimatter. Positrons are emitted from the decay of proton-rich radionuclides. Most of these radionu- clides are made in a cyclotron and have extremely short half-lives. The most commonly used PET radionuclide is fluorode- oxyglucose (FDG) labeled with fluorine-18 (a positron B Fig. 1.10 T1-weighted (A) and T2-weighted (B) MR images of the brain in the coronal plane. Fig. 1.11 A gamma camera. 9  The Body emitter). Tissues that are actively metabolizing glucose radiograph; that is, with the patient’s back closest to the take up this compound, and the resulting localized high X-ray tube.). concentration of this molecule compared to background Occasionally, when patients are too unwell to stand emission is detected as a “hot spot.” erect, films are obtained on the bed in an anteroposterior PET has become an important imaging modality in the (AP) position. These films are less standardized than PA detection of cancer and the assessment of its treatment films, and caution should always be taken when interpret- and recurrence. ing AP radiographs. The plain chest radiograph should always be Single photon emission computed tomography checked for quality. Film markers should be placed on the Single photon emission computed tomography (SPECT) appropriate side. (Occasionally patients have dextrocardia, is an imaging modality for detecting gamma rays which may be misinterpreted if the film marker is placed emitted from the decay of injected radionuclides such as inappropriately.) A good-quality chest radiograph will technetium-99m, iodine-123, or iodine-131. The rays are demonstrate the lungs, cardiomediastinal contour, dia- detected by a 360-degree rotating camera, which allows the phragm, ribs, and peripheral soft tissues. construction of 3D images. SPECT can be used to diagnose a wide range of disease conditions such as coronary artery Abdominal radiograph disease and bone fractures. Plain abdominal radiographs are obtained in the AP supine position. From time to time an erect plain abdominal IMAGE INTERPRETATION radiograph is obtained when small bowel obstruction is suspected. Imaging is necessary in most clinical specialties to diagnose pathological changes to tissues. It is paramount to appreci- Gastrointestinal contrast examinations ate what is normal and what is abnormal. An appreciation High-density contrast medium is ingested to opacify the of how the image is obtained, what the normal variations esophagus, stomach, small bowel, and large bowel. As are, and what technical considerations are necessary to described previously (p. 6), the bowel is insufflated with air obtain a radiological diagnosis. Without understanding the (or carbon dioxide) to provide a double-contrast study. In anatomy of the region imaged, it is impossible to comment many countries, endoscopy has superseded upper gastro- on the abnormal. intestinal imaging, but the mainstay of imaging the large bowel is the double-contrast barium enema. Typically the Plain radiography patient needs to undergo bowel preparation, in which Plain radiographs are undoubtedly the most common form powerful cathartics are used to empty the bowel. At the of image obtained in a hospital or local practice. Before time of the examination a small tube is placed into the interpretation, it is important to know about the imaging rectum and a barium suspension is run into the large technique and the views obtained as standard. bowel. The patient undergoes a series of twists and turns In most instances (apart from chest radiography) the so that the contrast passes through the entire large bowel. X-ray tube is 1 m away from the X-ray film. The object in The contrast is emptied and air is passed through the same question, for example a hand or a foot, is placed upon the tube to insufflate the large bowel. A thin layer of barium film. When describing subject placement for radiography, coats the normal mucosa, allowing mucosal detail to be the part closest to the X-ray tube is referred to first and that visualized (see Fig. 1.4). closest to the film is referred to second. For example, when positioning a patient for an anteroposterior (AP) radio- Urological contrast studies graph, the more anterior part of the body is closest to the Intravenous urography is the standard investigation for tube and the posterior part is closest to the film. assessing the urinary tract. Intravenous contrast medium When X-rays are viewed on a viewing box, the right side is injected, and images are obtained as the medium is of the patient is placed to the observer’s left; therefore, the excreted through the kidneys. A series of films are obtained observer views the radiograph as though looking at a during this period from immediately after the injection up patient in the anatomical position. to approximately 20 minutes later, when the bladder is full of contrast medium. Chest radiograph This series of radiographs demonstrates the kidneys, The chest radiograph is one of the most commonly ureters, and bladder and enables assessment of the retro- requested plain radiographs. An image is taken with the peritoneum and other structures that may press on the 10 patient erect and placed posteroanteriorly (PA chest urinary tract.  Imaging Safety in Imaging 1 and a series of representative films are obtained for Computed tomography clinical use. Computed tomography is the preferred terminology rather than computerized tomography, though both terms are SAFETY IN IMAGING used interchangeably by physicians. It is important for the student to understand the presen- Whenever a patient undergoes an X-ray or nuclear medi- tation of images. Most images are acquired in the axial cine investigation, a dose of radiation is given (Table 1.1). plane and viewed such that the observer looks from below As a general principle it is expected that the dose given is and upward toward the head (from the foot of the bed). By as low as reasonably possible for a diagnostic image to be implication: obtained. Numerous laws govern the amount of radiation exposure that a patient can undergo for a variety of proce- the right side of the patient is on the left side of the dures, and these are monitored to prevent any excess or image, and additional dosage. Whenever a radiograph is booked, the the uppermost border of the image is anterior. clinician ordering the procedure must appreciate its neces- sity and understand the dose given to the patient to ensure Many patients are given oral and intravenous contrast that the benefits significantly outweigh the risks. media to differentiate bowel loops from other abdominal Imaging modalities such as ultrasound and MRI are organs and to assess the vascularity of normal anatomical ideal because they do not impart significant risk to the structures. When intravenous contrast is given, the earlier patient. Moreover, ultrasound imaging is the modality of the images are obtained, the greater the likelihood of arte- choice for assessing the fetus. rial enhancement. As the time is delayed between injection Any imaging device is expensive, and consequently and image acquisition, a venous phase and an equilibrium the more complex the imaging technique (e.g., MRI) the phase are also obtained. more expensive the investigation. Investigations must be The great advantage of CT scanning is the ability to carried out judiciously, based on a sound clinical history extend and compress the gray scale to visualize the bones, and examination, for which an understanding of anatomy soft tissues, and visceral organs. Altering the window set- is vital. tings and window centering provides the physician with specific information about these structures. Magnetic resonance imaging There is no doubt that MRI has revolutionized the under- standing and interpretation of the brain and its coverings. Table 1.1 The approximate dosage of radiation exposure Furthermore, it has significantly altered the practice of as an order of magnitude musculoskeletal medicine and surgery. Images can be Typical Equivalent duration obtained in any plane and in most sequences. Typically the effective of background images are viewed using the same principles as CT. Intrave- Examination dose (mSv) exposure nous contrast agents are also used to further enhance tissue Chest radiograph 0.02 3 days contrast. Typically, MRI contrast agents contain paramag- Abdomen 1.00 6 months netic substances (e.g., gadolinium and manganese). Intravenous urography 2.50 14 months Nuclear medicine imaging CT scan of head 2.30 1 year CT scan of abdomen 10.00 4.5 years Most nuclear medicine images are functional studies. and pelvis Images are usually interpreted directly from a computer, 11  The Body Body systems SKELETAL SYSTEM The skeleton can be divided into two subgroups, the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones of the skull (cranium), vertebral column, ribs, and sternum, whereas the appendicular skeleton consists of the bones of the upper and lower limbs (Fig. 1.12). The skeletal system consists of cartilage and bone. Cartilage Cartilage is an avascular form of connective tissue consist- ing of extracellular fibers embedded in a matrix that con- tains cells localized in small cavities. The amount and kind of extracellular fibers in the matrix varies depending on the type of cartilage. In heavy weightbearing areas or areas prone to pulling forces, the amount of collagen is greatly increased and the cartilage is almost inextensible. In con- trast, in areas where weightbearing demands and stress are less, cartilage containing elastic fibers and fewer collagen fibers is common. The functions of cartilage are to: support soft tissues, provide a smooth, gliding surface for bone articulations at joints, and enable the development and growth of long bones. There are three types of cartilage: hyaline—most common; matrix contains a moderate amount of collagen fibers (e.g., articular surfaces of bones); elastic—matrix contains collagen fibers along with a large number of elastic fibers (e.g., external ear); Axial skeleton fibrocartilage—matrix contains a limited number of cells and ground substance amidst a substantial amount Appendicular skeleton of collagen fibers (e.g., intervertebral discs). Cartilage is nourished by diffusion and has no blood Fig. 1.12 The axial skeleton and the appendicular skeleton. vessels, lymphatics, or nerves. 12  Body Systems Skeletal System 1 Bone Bone is a calcified, living, connective tissue that forms the majority of the skeleton. It consists of an intercellular calcified matrix, which also contains collagen fibers, and several types of cells within the matrix. Bones function as: supportive structures for the body, protectors of vital organs, reservoirs of calcium and phosphorus, levers on which muscles act to produce movement, and containers for blood-producing cells. Os trigonum There are two types of bone, compact and spongy (tra- becular or cancellous). Compact bone is dense bone that forms the outer shell of all bones and surrounds spongy bone. Spongy bone consists of spicules of bone enclosing cavities containing blood-forming cells (marrow). Classifi- cation of bones is by shape. Long bones are tubular (e.g., humerus in upper limb; femur in lower limb). A Short bones are cuboidal (e.g., bones of the wrist and ankle). Sesamoid bones Flat bones consist of two compact bone plates separated by spongy bone (e.g., skull). Irregular bones are bones with various shapes (e.g., bones of the face). Sesamoid bones are round or oval bones that develop in tendons. In the clinic Accessory and sesamoid bones These are extra bones that are not usually found as part of the normal skeleton, but can exist as a normal variant in many people. They are typically found in multiple locations in the wrist and hands, ankles and feet (Fig. 1.13). These should not be mistaken for fractures on imaging. Sesamoid bones are embedded within tendons, the largest of which is the patella. There are many other sesamoids in the body particularly in tendons of the hands and feet, and most frequently in flexor tendons of the thumb and big toe. Degenerative and inflammatory changes of, as well as Os naviculare B mechanical stresses on, the accessory bones and sesamoids can cause pain, which can be treated with Fig. 1.13 Accessory and sesamoid bones. A. Radiograph of physiotherapy and targeted steroid injections, but in some the ankle region showing an accessory bone (os trigonum). severe cases it may be necessary to surgically remove the B. Radiograph of the feet showing numerous sesamoid bones and an accessory bone (os naviculare). bone. 13  The Body Bones are vascular and are innervated. Generally, an vessels that supply the bone and the periosteum. Most of adjacent artery gives off a nutrient artery, usually one per the nerves passing into the internal cavity with the nutrient bone, that directly enters the internal cavity of the bone artery are vasomotor fibers that regulate blood flow. Bone and supplies the marrow, spongy bone, and inner layers of itself has few sensory nerve fibers. On the other hand, the compact bone. In addition, all bones are covered externally, periosteum is supplied with numerous sensory nerve fibers except in the area of a joint where articular cartilage is and is very sensitive to any type of injury. present, by a fibrous connective tissue membrane called the Developmentally, all bones come from mesenchyme by periosteum, which has the unique capability of forming new either intramembranous ossification, in which mesenchy- bone. This membrane receives blood vessels whose branches mal models of bones undergo ossification, or endochondral supply the outer layers of compact bone. A bone stripped ossification, in which cartilaginous models of bones form of its periosteum will not survive. Nerves accompany the from mesenchyme and undergo ossification. In the clinic Determination of skeletal age Throughout life the bones develop in a predictable way to form the skeletally mature adult at the end of puberty. In western countries skeletal maturity tends to occur between the ages of 20 and 25 years. However, this may well vary according to geography and socioeconomic conditions. Skeletal maturity will also be determined by genetic factors and disease states. Up until the age of skeletal maturity, bony growth and development follows a typically predictable ordered state, which can be measured through either ultrasound, plain radiographs, or MRI scanning. Typically, the nondominant (left) hand is radiographed, and the radiograph is compared A B to a series of standard radiographs. From these images the bone age can be determined (Fig. 1.14). In certain disease states, such as malnutrition and hypothyroidism, bony maturity may be slow. If the skeletal bone age is significantly reduced from the patient’s true age, treatment may be required. In the healthy individual the bone age accurately represents the true age of the patient. This is important in determining the true age of the subject. This may also have medicolegal importance. Carpal bones C D Fig. 1.14 A developmental series of radiographs showing the progressive ossification of carpal (wrist) bones from 3 (A) to 10 (D) years of age. 14  Body Systems Skeletal System 1 In the clinic Bone marrow transplants Red marrow in body of lumbar vertebra The bone marrow serves an important function. There are two types of bone marrow, red marrow (otherwise known as myeloid tissue) and yellow marrow. Red blood cells, platelets, and most white blood cells arise from within the red marrow. In the yellow marrow a few white cells are made; however, this marrow is dominated by large fat globules (producing its yellow appearance) (Fig. 1.15). From birth most of the body’s marrow is red; however, as the subject ages, more red marrow is converted into yellow marrow within the medulla of the long and flat bones. Bone marrow contains two types of stem cells. Hemopoietic stem cells give rise to the white blood cells, red blood cells, and platelets. Mesenchymal stem cells differentiate into structures that form bone, cartilage, and muscle. There are a number of diseases that may involve the bone marrow, including infection and malignancy. In patients who develop a bone marrow malignancy (e.g., leukemia) it may be possible to harvest nonmalignant cells from the patient’s bone marrow or cells from another person’s bone marrow. The patient’s own marrow can be destroyed with Yellow marrow in femoral head chemotherapy or radiation and the new cells infused. This treatment is bone marrow transplantation. Fig. 1.15 T1-weighted image in the coronal plane, demonstrating the relatively high signal intensity returned from the femoral heads and proximal femoral necks, consistent with yellow marrow. In this young patient, the vertebral bodies return an intermediate darker signal that represents red marrow. There is relatively little fat in these vertebrae; hence the lower signal return. 15  The Body In the clinic Bone fractures Fractures occur in normal bone because of abnormal load or stress, in which the bone gives way (Fig. 1.16A). Fractures may also occur in bone that is of poor quality (osteoporosis); in such cases a normal stress is placed upon a bone that is not of sufficient quality to withstand this force and subsequently fractures. In children whose bones are still developing, fractures may occur across the growth plate or across the shaft. These shaft fractures typically involve partial cortical disruption, similar to breaking a branch of a young tree; hence they are A termed “greenstick” fractures. After a fracture has occurred, the natural response is to heal the fracture. Between the fracture margins a blood clot is formed into which new vessels grow. A jelly-like matrix is formed, and further migration of collagen-producing cells occurs. On this soft tissue framework, calcium hydroxyapatite is produced by osteoblasts and forms insoluble crystals, and then bone matrix is laid down. As more bone is produced, a callus can be demonstrated forming across the fracture site. B Treatment of fractures requires a fracture line reduction. If this cannot be maintained in a plaster of Paris cast, it may Fig. 1.16 Radiograph, lateral view, showing fracture of the ulna require internal or external fixation with screws and metal at the elbow joint (A) and repair of this fracture (B) using rods (Fig. 1.16B). internal fixation with a plate and multiple screws. In the clinic Avascular necrosis Wasting of gluteal muscle Avascular necrosis is cellular death of bone resulting from a temporary or permanent loss of blood supply to that bone. Avascular necrosis may occur in a variety of medical conditions, some of which have an etiology that is less than clear. A typical site for avascular necrosis is a fracture across the femoral neck in an elderly patient. In these patients there is loss of continuity of the cortical medullary blood flow with loss of blood flow deep to the retinacular fibers. This essentially renders the femoral head bloodless; it subsequently undergoes necrosis and collapses (Fig. 1.17). In these patients it is necessary to replace the femoral head with a prosthesis. Avascular necrosis Bladder Normal left hip Fig. 1.17 Image of the hip joints demonstrating loss of height of the right femoral head with juxta-articular bony sclerosis and subchondral cyst formation secondary to avascular necrosis. There is also significant wasting of the muscles supporting the hip, which is secondary to disuse and pain. 16  Body Systems Skeletal System 1 In the clinic Epiphyseal fractures As the skeleton develops, there are stages of intense growth typically around the ages of 7 to 10 years and later in puberty. These growth spurts are associated with increased cellular activity around the growth plate between the head and shaft of a bone. This increase in activity renders the growth plates more vulnerable to injuries, which may occur from dislocation across a Bone Articular cavity Bone growth plate or fracture through a growth plate. A Synovial joint Occasionally an injury may result in growth plate compression, destroying that region of the growth plate, which may result in asymmetrical growth across that joint region. All fractures across the growth plate must be treated with care and expediency, requiring fracture reduction. Bone Connective tissue Bone B Solid joint Joints Fig. 1.18 Joints. A. Synovial joint. B. Solid joint. The sites where two skeletal elements come together are termed joints. The two general categories of joints The synovial membrane attaches to the margins of the (Fig. 1.18) are those in which: joint surfaces at the interface between the cartilage and bone and encloses the articular cavity. The synovial the skeletal elements are separated by a cavity (i.e., membrane is highly vascular and produces synovial synovial joints), and fluid, which percolates into the articular cavity and there is no cavity and the components are held together lubricates the articulating surfaces. Closed sacs of by connective tissue (i.e., solid joints). synovial membrane also occur outside joints, where they form synovial bursae or tendon sheaths. Bursae Blood vessels that cross over a joint and nerves that often intervene between structures, such as tendons innervate muscles acting on a joint usually contribute and bone, tendons and joints, or skin and bone, and articular branches to that joint. reduce the friction of one structure moving over the other. Tendon sheaths surround tendons and also Synovial joints reduce friction. Synovial joints are connections between skeletal compo- The fibrous membrane is formed by dense connective nents where the elements involved are separated by a tissue and surrounds and stabilizes the joint. Parts of narrow articular cavity (Fig. 1.19). In addition to contain- the fibrous membrane may thicken to form ligaments, ing an articular cavity, these joints have a number of which further stabilize the joint. Ligaments outside the characteristic features. capsule usually provide additional reinforcement. First, a layer of cartilage, usually hyaline cartilage, covers the articulating surfaces of the skeletal elements. In other words, bony surfaces do not normally contact one Another common but not universal feature of synovial another directly. As a consequence, when these joints are joints is the presence of additional structures within the viewed in normal radiographs, a wide gap seems to sepa- area enclosed by the capsule or synovial membrane, such rate the adjacent bones because the cartilage that covers as articular discs (usually composed of fibrocartilage), the articulating surfaces is more transparent to X-rays fat pads, and tendons. Articular discs absorb compres- than bone. sion forces, adjust to changes in the contours of joint sur- A second characteristic feature of synovial joints is the faces during movements, and increase the range of presence of a joint capsule consisting of an inner syno- movements that can occur at joints. Fat pads usually occur vial membrane and an outer fibrous membrane. between the synovial membrane and the capsule and move 17  The Body Tendon Sheath Synovial Hyaline cartilage membrane Fat pad Joint capsule Articular cavity Fibrous Articular membrane disc Bone Bone Hyaline cartilage Bone Articular cavity Bone Fibrous membrane Synovial Skin Bursa membrane A B Fig. 1.19 Synovial joints. A. Major features of a synovial joint. B. Accessory structures associated with synovial joints. into and out of regions as joint contours change during bicondylar (two sets of contact points), condylar (ellip- movement. Redundant regions of the synovial membrane soid), saddle, and ball and socket; and fibrous membrane allow for large movements at joints. based on movement, synovial joints are described as uniaxial (movement in one plane), biaxial (movement Descriptions of synovial joints based on shape in two planes), and multiaxial (movement in three and movement planes). Synovial joints are described based on shape and movement: Hinge joints are uniaxial, whereas ball and socket joints are multiaxial. based on the shape of their articular surfaces, synovial joints are described as plane (flat), hinge, pivot, 18  Body Systems Skeletal System 1 adduction, circumduction, and rotation (e.g., hip Specific types of synovial joints joint) (Fig. 1.20) Plane joints—allow sliding or gliding movements when Solid joints one bone moves across the surface of another (e.g., Solid joints are connections between skeletal elements acromioclavicular joint) where the adjacent surfaces are linked together either Hinge joints—allow movement around one axis that by fibrous connective tissue or by cartilage, usually fibro- passes transversely through the joint; permit flexion and cartilage (Fig. 1.21). Movements at these joints are more extension (e.g., elbow [humero-ulnar] joint) restricted than at synovial joints. Pivot joints—allow movement around one axis that Fibrous joints include sutures, gomphoses, and passes longitudinally along the shaft of the bone; permit syndesmoses. rotation (e.g., atlanto-axial joint) Bicondylar joints—allow movement mostly in one axis Sutures occur only in the skull where adjacent bones with limited rotation around a second axis; formed by are linked by a thin layer of connective tissue termed a two convex condyles that articulate with concave or flat sutural ligament. surfaces (e.g., knee joint) Gomphoses occur only between the teeth and adjacent Condylar (ellipsoid) joints—allow movement around bone. In these joints, short collagen tissue fibers in the two axes that are at right angles to each other; permit periodontal ligament run between the root of the tooth flexion, extension, abduction, adduction, and circum- and the bony socket. duction (limited) (e.g., wrist joint) Syndesmoses are joints in which two adjacent bones Saddle joints—allow movement around two axes that are linked by a ligament. Examples are the ligamentum are at right angles to each other; the articular surfaces flavum, which connects adjacent vertebral laminae, are saddle shaped; permit flexion, extension, abduction, and an interosseous membrane, which links, for adduction, and circumduction (e.g., carpometacarpal example, the radius and ulna in the forearm. joint of the thumb) Ball and socket joints—allow movement around Cartilaginous joints include synchondroses and multiple axes; permit flexion, extension, abduction, symphyses. B Humerus Ulna Radius Synovial membrane Wrist joint Articular disc Radius Olecranon A Synovial cavity C Ulna Odontoid process Cartilage of axis Trapezium Synovial membrane Atlas Metacarpal I Synovial Femur membrane D E F Fig. 1.20 Various types of synovial joints. A. Condylar (wrist). B. Gliding (radio-ulnar). C. Hinge (elbow). D. Ball and socket (hip). E. Saddle 19 (carpometacarpal of thumb). F. Pivot (atlanto-axial).  The Body SOLID JOINTS Fibrous Cartilaginous Sutures Sutural ligament Skull Synchondrosis Head Gomphosis Cartilage of growth plate Tooth Long bone Shaft Periodontal ligament Bone Symphysis Intervertebral Syndesmosis discs Radius Ulna Interosseous membrane Pubic symphysis Fig. 1.21 Solid joints. Synchondroses occur where two ossification centers Symphyses occur where two separate bones are inter- in a developing bone remain separated by a layer of connected by cartilage. Most of these types of joints cartilage, for example, the growth plate that occurs occur in the midline and include the pubic symphysis between the head and shaft of developing long bones. between the two pelvic bones, and intervertebral discs These joints allow bone growth and eventually become between adjacent vertebrae. 20 completely ossified.  Body Systems Skeletal System 1 In the clinic Degenerative joint disease In the United States, osteoarthritis accounts for up to Degenerative joint disease is commonly known as one-quarter of primary health care visits and is regarded as a osteoarthritis or osteoarthrosis. The disorder is related to significant problem. aging but not caused by aging. Typically there are decreases The etiology of osteoarthritis is not clear; however, in water and proteoglycan content within the cartilage. The osteoarthritis can occur secondary to other joint diseases, cartilage becomes more fragile and more susceptible to such as rheumatoid arthritis and infection. Overuse of joints mechanical disruption (Fig. 1.22). As the cartilage wears, the and abnormal strains, such as those experienced by people underlying bone becomes fissured and also thickens. who play sports, often cause one to be more susceptible to Synovial fluid may be forced into small cracks that appear in chronic joint osteoarthritis. the bone’s surface, which produces large cysts. Furthermore, Various treatments are available, including weight reactive juxta-articular bony nodules are formed reduction, proper exercise, anti-inflammatory drug treatment, (osteophytes) (Fig. 1.23). As these processes occur, there is and joint replacement (Fig. 1.24). slight deformation, which alters the biomechanical forces through the joint. This in turn creates abnormal stresses, which further disrupt the joint. Osteophytes Cartilage loss Patella Femoral condyles Cartilage loss Loss of joint space Fig. 1.22 This operative photograph demonstrates the focal Fig. 1.23 This radiograph demonstrates the loss of joint space in areas of cartilage loss in the patella and femoral condyles the medial compartment and presence of small spiky throughout the knee joint. osteophytic regions at the medial lateral aspect of the joint. 21  The Body In the clinic—cont’d Arthroscopy Arthroscopy is a technique of visualizing the inside of a joint using a small telescope placed through a tiny incision in the skin. Arthroscopy can be performed in most joints. However, it is most commonly performed in the knee, shoulder, ankle, and hip joints. Arthroscopy allows the surgeon to view the inside of the joint and its contents. Notably, in the knee, the menisci and the ligaments are easily seen, and it is possible using separate puncture sites and specific instruments to remove the menisci and replace the cruciate ligaments. The advantages of arthroscopy are that it is performed through small incisions, it enables patients to quickly recover and return to normal activity, and it only requires either a light anesthetic or regional anesthesia during the procedure. Fig. 1.24 After knee replacement. This radiograph shows the position of the prosthesis. In the clinic Joint replacement Joint replacement is undertaken for a variety of reasons. These predominantly include degenerative joint disease and joint destruction. Joints that have severely degenerated or lack their normal function are painful. In some patients, the pain may be so severe that it prevents them from leaving the house and undertaking even the smallest of activities without discomfort. Large joints are commonly affected, including the hip, knee, and shoulder. However, with ongoing developments in joint replacement materials and surgical techniques, even small joints of the fingers can be replaced. Typically, both sides of the joint are replaced; in the hip joint the acetabulum will be reamed, and a plastic or metal cup will be introduced. The femoral component will be fitted precisely to the femur and cemented in place (Fig. 1.25). Most patients derive significant benefit from joint replacement and continue to lead an active life afterward. In a minority of patients who have been fitted with a metal acetabular cup and metal femoral component, an aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL) may develop, possibly caused by a hypersensitivity response to the release of metal ions in adjacent tissues. These Artificial femoral head Acetabulum patients often have chronic pain and might need additional surgery to replace these joint replacements with safer Fig. 1.25 This is a radiograph, anteroposterior view, of the models. pelvis after a right total hip replacement. There are additional significant degenerative changes in the left hip joint, which will also need to be replaced. 22  Body Systems Muscular System 1 SKIN AND FASCIAS In the clinic Skin The importance of fascias The skin is the largest organ of the body. It consists of the A fascia is a thin band of tissue that surrounds muscles, epidermis and the dermis. The epidermis is the outer cel- bones, organs, nerves, and blood vessels and often remains uninterrupted as a 3D structure between tissues. It lular layer of stratified squamous epithelium, which is provides important support for tissues and can provide a avascular and varies in thickness. The dermis is a dense bed boundary between structures. of vascular connective tissue. Clinically, fascias are extremely important because they The skin functions as a mechanical and permeability often limit the spread of infection and malignant disease. barrier, and as a sensory and thermoregulatory organ. It When infections or malignant diseases cross a fascial also can initiate primary immune responses. plain, a primary surgical clearance may require a far more extensive dissection to render the area free of tumor or infection. Fascia A typical example of the clinical importance of a fascial Fascia is connective tissue containing varying amounts of layer would be of that covering the psoas muscle. fat that separate, support, and interconnect organs and Infection within an intervertebral body secondary to structures, enable movement of one structure relative to tuberculosis can pass laterally into the psoas muscle. Pus fills the psoas muscle but is limited from further spread by another, and allow the transit of vessels and nerves from the psoas fascia, which surrounds the muscle and extends one area to another. There are two general categories of inferiorly into the groin pointing below the inguinal fascia: superficial and deep. ligament. Superficial (subcutaneous) fascia lies just deep to and is attached to the dermis of the skin. It is made up of loose In the clinic connective tissue usually containing a large amount of fat. The thickness of the superficial fascia (subcutane- Placement of skin incisions and scarring ous tissue) varies considerably, both from one area of Surgical skin incisions are ideally placed along or parallel to Langer’s lines, which are lines of skin tension that the body to another and from one individual to another. correspond to the orientation of the dermal collagen The superficial fascia allows movement of the skin over fibers. They tend to run in the same direction as the deeper areas of the body, acts as a conduit for vessels and underlying muscle fibers and incisions that are made nerves coursing to and from the skin, and serves as an along these lines tend to heal better with less scarring. In energy (fat) reservoir. contrast, incisions made perpendicular to Langer’s lines Deep fascia usually consists of dense, organized connec- are more likely to heal with a prominent scar and in some tive tissue. The outer layer of deep fascia is attached to severe cases can lead to raised, firm, hypertrophic, or the deep surface of the superficial fascia and forms a keloid, scars. thin fibrous covering over most of the deeper region of the body. Inward extensions of this fascial layer form MUSCULAR SYSTEM intermuscular septa that compartmentalize groups of muscles with similar functions and innervations. Other The muscular system is generally regarded as consisting of extensions surround individual muscles and groups of one type of muscle found in the body—skeletal muscle. vessels and nerves, forming an investing fascia. Near However, there are two other types of muscle tissue found some joints the deep fascia thickens, forming retinacula. in the body, smooth muscle and cardiac muscle, that are These fascial retinacula hold tendons in place and important components of other systems. These three types prevent them from bowing during movements at the of muscle can be characterized by whether they are con- joints. Finally, there is a layer of deep fascia separating trolled voluntarily or involuntarily, whether they appear the membrane lining the abdominal cavity (the parietal striated (striped) or smooth, and whether they are associ- peritoneum) from the fascia covering the deep surface ated with the body wall (somatic) or with organs and blood of the muscles of the abdominal wall (the transversalis vessels (visceral). fascia). This layer is referred to as extraperitoneal fascia. A similar layer of fascia in the thorax is termed Skeletal muscle forms the majority of the muscle tissue the endothoracic fascia. in the body. It consists of parallel bundles of long, 23  The Body multinucleated fibers with transverse stripes, is capable In the clinic of powerful contractions, and is innervated by somatic and branchial motor nerves. This muscle is used to Muscle paralysis move bones and other structures, and provides support Muscle paralysis is the inability to move a specific muscle and gives form to the body. Individual skeletal muscles or muscle group and may be associated with other neurological abnormalities, including loss of sensation. are often named on the basis of shape (e.g., rhomboid Major causes include stroke, trauma, poliomyelitis, and major muscle), attachments (e.g., sternohyoid muscle), iatrogenic factors. Paralysis may be due to abnormalities function (e.g., flexor pollicis longus muscle), position in the brain, the spinal cord, and the nerves supplying the (e.g., palmar interosseous muscle), or fiber orientation muscles. (e.g., external oblique muscle). In the long term, muscle paralysis will produce Cardiac muscle is striated muscle found only in the walls secondary muscle wasting and overall atrophy of the of the heart (myocardium) and in some of the large region due to disuse. vessels close to where they join the heart. It consists of a branching network of individual cells linked electri- cally and mechanically to work as a unit. Its contrac- tions are less powerful than those of skeletal muscle and it is resistant to fatigue. Cardiac muscle is innervated by In the clinic visceral motor nerves. Smooth muscle (absence of stripes) consists of elongated Muscle atrophy or spindle-shaped fibers capable of slow and sustained Muscle atrophy is a wasting disorder of muscle. It can be contractions. It is found in the walls of blood vessels produced by a variety of causes, which include nerve damage to the muscle and disuse. (tunica media), associated with hair follicles in the skin, Muscle atrophy is an important problem in patients located in the eyeball, and found in the walls of various who have undergone long-term rest or disuse, requiring structures associated with the gastrointestinal, respira- extensive rehabilitation and muscle building exercises to tory, genitourinary, and urogenital systems. Smooth maintain normal activities of daily living. muscle is innervated by visceral motor nerves. In the clinic Muscle injuries and strains identify which muscle groups are affected and the extent of Muscle injuries and strains tend to occur in specific muscle the tear to facilitate treatment and obtain a prognosis, which groups and usually are related to a sudden exertion and will determine the length of rehabilitation necessary to muscle disruption. They typically occur in athletes. return to normal activity. Muscle tears may involve a small interstitial injury up to a complete muscle disruption (Fig. 1.26). It is important to Fig. 1.26 Axial inversion recovery MR imaging series, which suppresses fat and soft tissue and leaves high signal intensity where fluid is seen. A muscle tear in the right adductor longus with edema in and around the muscle is shown. Torn right adductor longus Normal left adductor longus 24  Body Systems Cardiovascular System 1 CARDIOVASCULAR SYSTEM Examples of large veins are the superior vena cava, the inferior vena cava, and the portal vein. The cardiovascular system consists of the heart, which Small and medium veins contain small amounts of pumps blood throughout the body, and the blood vessels, smooth muscle, and the thickest layer is the tunica which are a closed network of tubes that transport the externa. Examples of small and medium veins are blood. There are three types of blood vessels: superficial veins in the upper and lower limbs and deeper veins of the leg and forearm. arteries, which transport blood away from the heart; Venules are the smallest veins and drain the veins, which transport blood toward the heart; capillaries. capillaries, which connect the arteries and veins, are the smallest of the blood vessels and are where oxygen, Although veins are similar in general structure to arter- nutrients, and wastes are exchanged within the tissues. ies, they have a number of distinguishing features. The walls of the blood vessels of the cardiovascular The walls of veins, specifically the tunica media, are system usually consist of three layers or tunics: thin. The luminal diameters of veins are large. tunica externa (adventitia)—the outer connective tissue There often are multiple veins (venae comitantes) closely layer, associated with arteries in peripheral regions. tunica media—the middle smooth muscle layer (may Valves often are present in veins, particularly in periph- also contain varying amounts of elastic fibers in medium eral vessels inferior to the level of the heart. These are and large arteries), and usually paired cusps that facilitate blood flow toward tunica intima—the inner endothelial lining of the blood the heart. vessels. More specific information about the cardiovascular Arteries are usually further subdivided into three system and how it relates to the circulation of blood classes, according to the variable amounts of smooth throughout the body will be discussed, where appropriate, muscle and elastic fibers contributing to the thickness of in each of the succeeding chapters of the text. the tunica media, the overall size of the vessel, and its function. Large elastic arteries contain substantial amounts of elastic fibers in the tunica media, allowing expansion and recoil during the normal cardiac cycle. This helps In the clinic maintain a constant flow of blood during diastole. Examples of large elastic arteries are the aorta, the Atherosclerosis brachiocephalic trunk, the left common carotid artery, Atherosclerosis is a disease that affects arteries. There is a the left subclavian artery, and the pulmonary trunk. chronic inflammatory reaction in the walls of the arteries, with deposition of cholesterol and fatty proteins. This may Medium muscular arteries are composed of a tunica in turn lead to secondary calcification, with reduction in media that contains mostly smooth muscle fibers. This the diameter of the vessels impeding distal flow. The characteristic allows these vessels to regulate their plaque itself may be a site for attraction of platelets that diameter and control the flow of blood to different parts may “fall off” (embolize) distally. Plaque fissuring may of the body. Examples of medium muscular arteries are occur, which allows fresh clots to form and occlude the most of the named arteries, including the femoral, axil- vessel. lary, and radial arteries. The importance of atherosclerosis and its effects Small arteries and arterioles control the filling of the depend upon which vessel is affected. If atherosclerosis capillaries and directly contribute to the arterial pres- occurs in the carotid artery, small emboli may form and sure in the vascular system. produce a stroke. In the heart, plaque fissuring may produce an acute vessel thrombosis, producing a myocardial infarction (heart attack). In the legs, chronic Veins also are subdivided into three classes. narrowing of vessels may limit the ability of the patient to walk and ultimately cause distal ischemia and gangrene of Large veins contain some smooth muscle in the tunica the toes. media, but the thickest layer is the tunica externa. 25  The Body In the clinic Varicose veins Varicose veins Varicose veins are tortuous dilated veins that typically occur in the legs, although they may occur in the superficial veins of the arm and in other organs. In normal individuals the movement of adjacent leg muscles pumps the blood in the veins to the heart. Blood is also pumped from the superficial veins through the investing layer of fascia of the leg into the deep veins. Valves in these perforating veins may become damaged, allowing blood to pass in the opposite direction. This increased volume and pressure produces dilatation and tortuosity of the superficial veins (Fig. 1.27). Apart from the unsightliness of larger veins, the skin may become pigmented and atrophic with a poor response to tissue trauma. In some patients even small trauma may produce skin ulceration, which requires elevation of the limb and application of pressure bandages to heal. Treatment of varicose veins depends on their location, size, and severity. Typically the superficial varicose veins can be excised and stripped, allowing blood only to drain into the deep system. Fig. 1.27 Photograph demonstrating varicose veins. In the clinic Anastomoses and collateral circulation considerable problem in patients who have undergone portal All organs require a blood supply from the arteries and vein thrombosis or occlusion, where venous drainage from drainage by veins. Within most organs there are multiple the gut bypasses the liver through collateral veins to return ways of perfusing the tissue such that if the main vessel to the systemic circulation. feeding the organ or vein draining the organ is blocked, a Normal vascular anastomoses associated with an organ series of smaller vessels (collateral vessels) continue to are important. Some organs, such as the duodenum, have a supply and drain the organ. dual blood supply arising from the branches of the celiac In certain circumstances, organs have more than one trunk and also from the branches of the superior mesenteric vessel perfusing them, such as the hand, which is supplied artery. Should either of these vessels be damaged, blood by the radial and ulnar arteries. Loss