Snell's (9th Ed) Basic Anatomy PDF

2 Chapter 1 Introduction Basic Anatomy cannot accurately discuss or record the abnormal f­ unctions of joints, the actions of muscles, the alteration of position of...

2 Chapter 1 Introduction Basic Anatomy cannot accurately discuss or record the abnormal f­ unctions of joints, the actions of muscles, the alteration of position of organs, or the exact location of swellings or tumors. Anatomy is the science of the structure and function of the body. Clinical anatomy is the study of the macroscopic struc- Terms Related to Position ture and function of the body as it relates to the practice of All descriptions of the human body are based on the medicine and other health sciences. assumption that the person is standing erect, with the upper Basic anatomy is the study of the minimal amount of limbs by the sides and the face and palms of the hands anatomy consistent with the understanding of the overall directed forward (Fig. 1.1). This is the so-called anatomic structure and function of the body. position. The various parts of the body are then described in relation to certain imaginary planes. Descriptive Anatomic Terms Median Sagittal Plane It is important for medical personnel to have a sound This is a vertical plane passing through the center of knowledge and understanding of the basic anatomic terms. the body, dividing it into equal right and left halves With the aid of a medical dictionary, you will find that (see Fig. 1.1). Planes situated to one or the other side of understanding anatomic terminology greatly assists you in the median plane and parallel to it are termed paramedian. the learning process. A structure situated nearer to the median plane of the body The accurate use of anatomic terms by medical person- than another is said to be medial to the other. Similarly, a nel enables them to communicate with their colleagues both structure that lies farther away from the median plane than nationally and internationally. Without anatomic terms, one another is said to be lateral to the other. median sagittal plane coronal plane median sagittal plane superior paramedian plane proximal end of upper limb horizontal or transverse lateral plane border posterior anterior dorsal distal surface end of of hand palmar upper surface limb of hand medial border dorsal surface of foot inferior plantar surface of foot FIGURE 1.1 Anatomic terms used in relation to position. Note that the subjects are standing in the anatomic position.  Basic Anatomy 3 Coronal Planes Rotation is the term applied to the movement of a These planes are imaginary vertical planes at right angles to part of the body around its long axis. Medial rotation is the median plane (see Fig. 1.1). the movement that results in the anterior surface of the part facing medially. Lateral rotation is the movement Horizontal, or Transverse, Planes that results in the anterior surface of the part facing These planes are at right angles to both the median and the laterally. coronal planes (see Fig. 1.1). Pronation of the forearm is a medial rotation of the The terms anterior and posterior are used to indicate forearm in such a manner that the palm of the hand faces the front and back of the body, respectively (see Fig. 1.1). posteriorly (see Fig. 1.3). Supination of the forearm is a To describe the relationship of two structures, one is said to lateral rotation of the forearm from the pronated posi- be anterior or posterior to the other insofar as it is closer to tion so that the palm of the hand comes to face anteriorly the anterior or posterior body surface. (see Fig. 1.3). In describing the hand, the terms palmar and dorsal Circumduction is the combination in sequence of the surfaces are used in place of anterior and posterior, and in movements of flexion, extension, abduction, and adduc- describing the foot, the terms plantar and dorsal surfaces tion (see Fig. 1.2). are used instead of lower and upper surfaces (see Fig. 1.1). Protraction is to move forward; retraction is to move The terms proximal and distal describe the relative distances backward (used to describe the forward and backward from the roots of the limbs; for example, the arm is proximal movement of the jaw at the temporomandibular joints). to the forearm and the hand is distal to the forearm. Inversion is the movement of the foot so that the sole The terms superficial and deep denote the relative faces in a medial direction (see Fig. 1.3). Eversion is the distances of structures from the surface of the body, and opposite movement of the foot so that the sole faces in a the terms superior and inferior denote levels relatively lateral direction (see Fig. 1.3). high or low with reference to the upper and lower ends of the body. The terms internal and external are used to describe the relative distance of a structure from the center of an organ Basic Structures or cavity; for example, the internal carotid artery is found Skin inside the cranial cavity and the external carotid artery is found outside the cranial cavity. The skin is divided into two parts: the superficial part, the The term ipsilateral refers to the same side of the body; epidermis; and the deep part, the dermis (Fig. 1.4). The for example, the left hand and the left foot are ipsilateral. epidermis is a stratified epithelium whose cells become flat- Contralateral refers to opposite sides of the body; for tened as they mature and rise to the surface. On the palms of example, the left biceps brachii muscle and the right rectus the hands and the soles of the feet, the epidermis is extremely femoris muscle are contralateral. thick, to withstand the wear and tear that occurs in these The supine position of the body is lying on the back. regions. In other areas of the body, for example, on the ante- The prone position is lying face downward. rior surface of the arm and forearm, it is thin. The dermis is composed of dense connective tissue containing many blood Terms Related to Movement vessels, lymphatic vessels, and nerves. It shows considerable variation in thickness in different parts of the body, tending A site where two or more bones come together is known to be thinner on the anterior than on the posterior surface. as a joint. Some joints have no movement (sutures of the It is thinner in women than in men. The dermis of the skin skull), some have only slight movement (superior tibiofib- is connected to the underlying deep fascia or bones by the ular joint), and some are freely movable (shoulder joint). superficial fascia, otherwise known as subcutaneous tissue. Flexion is a movement that takes place in a sagittal The skin over joints always folds in the same place, the plane. For example, flexion of the elbow joint approxi- SKIN CREASES (Fig. 1.5). At these sites, the skin is thinner mates the anterior surface of the forearm to the anterior than elsewhere and is firmly tethered to underlying struc- surface of the arm. It is usually an anterior movement, but tures by strong bands of fibrous tissue. it is occasionally posterior, as in the case of the knee joint The appendages of the skin are the nails, hair follicles, (see Fig. 1.2). Extension means straightening the joint and sebaceous glands, and sweat glands. usually takes place in a posterior direction (see Fig. 1.2). The nails are keratinized plates on the dorsal surfaces of Lateral flexion is a movement of the trunk in the coronal the tips of the fingers and toes. The proximal edge of the plane (Fig. 1.3). plate is the root of the nail (see Fig. 1.5). With the exception Abduction is a movement of a limb away from the mid- of the distal edge of the plate, the nail is surrounded and line of the body in the coronal plane (see Fig. 1.2). overlapped by folds of skin known as nail folds. The sur- Adduction is a movement of a limb toward the body in the face of skin covered by the nail is the nail bed (see Fig. 1.5). coronal plane (see Fig. 1.2). In the fingers and toes, abduction Hairs grow out of follicles, which are invaginations is applied to the spreading of these structures and adduction of the epidermis into the dermis (see Fig. 1.4). The folli- is applied to the drawing together of these structures (see cles lie obliquely to the skin surface, and their expanded Fig. 1.3). The movements of the thumb (see Fig. 1.3), which extremities, called hair bulbs, penetrate to the deeper part are a little more complicated, are described on page 413. of the dermis. Each hair bulb is concave at its end, and the 4 Chapter 1 Introduction abduction of shoulder adduction extension flexion of shoulder joint of hip joint abduction flexion adduction flexion of knee joint of elbow joint extension extension medial rotation circumduction of shoulder joint of shoulder joint lateral rotation of shoulder joint FIGURE 1.2 Some anatomic terms used in relation to movement. Note the difference between flexion of the elbow and that of the knee. c­ oncavity is occupied by vascular connective tissue called hair to move into a more vertical position; it also com- hair papilla. A band of smooth muscle, the arrector pili, presses the sebaceous gland and causes it to extrude some connects the undersurface of the follicle to the superficial of its secretion. The pull of the muscle also causes dimpling part of the dermis (see Fig. 1.4). The muscle is innervated of the skin surface, so-called gooseflesh. Hairs are dis- by sympathetic nerve fibers, and its contraction causes the tributed in various numbers over the whole surface of the  Basic Anatomy 5 lateral flexion of trunk supination of forearm pronation of forearm inversion of foot eversion of foot adduction of fingers abduction of fingers adduction of thumb opposition of thumb and little finger flexion of thumb abduction of thumb extension of thumb FIGURE 1.3 Additional anatomic terms used in relation to movement. body, except on the lips, the palms of the hands, the sides of the emerging hair. It also oils the surface epidermis of the fingers, the glans penis and clitoris, the labia minora around the mouth of the follicle. and the internal surface of the labia majora, and the soles Sweat glands are long, spiral, tubular glands distributed and sides of the feet and the sides of the toes. over the surface of the body, except on the red margins of Sebaceous glands pour their secretion, the sebum, onto the lips, the nail beds, and the glans penis and clitoris (see the shafts of the hairs as they pass up through the necks Fig. 1.4). These glands extend through the full thickness of of the follicles. They are situated on the sloping undersur- the dermis, and their extremities may lie in the superficial face of the follicles and lie within the dermis (see Fig. 1.4). fascia. The sweat glands are therefore the most deeply pen- Sebum is an oily material that helps preserve the flexibility etrating structures of all the epidermal appendages. 6 Chapter 1 Introduction shaft of hair plexus of arteries and veins sebaceous gland hair follicle arrector pili muscle plexus of arteries and veins nail folds superficial fascia nail root nail hair bulb body of eccrine sweat gland nail bed duct of eccrine sweat gland terminal phalanx FIGURE 1.4 General structure of the skin and its relationship to the superficial fascia. Note that hair follicles extend down FIGURE 1.5 The various skin creases on the palmar surface into the deeper part of the dermis or even into the super- of the hand and the anterior surface of the wrist joint. The ficial fascia, whereas sweat glands extend deeply into the relationship of the nail to other structures of the finger is superficial fascia. also shown. C L I N I C A L N O T E S Skin Infections f­ollicles, sebaceous glands, and sweat glands as well as from The nail folds, hair follicles, and sebaceous glands are com- the cells at the edge of the burn. A burn that extends deeper than mon sites for entrance into the underlying tissues of pathogenic the sweat glands heals slowly and from the edges only, and con- organisms such as Staphylococcus aureus. Infection occurring siderable contracture will be caused by fibrous tissue. To speed between the nail and the nail fold is called a paronychia. Infection up healing and reduce the incidence of contracture, a deep burn of the hair follicle and sebaceous gland is responsible for the com- should be grafted. mon boil. A carbuncle is a staphylococcal infection of the superfi- cial fascia. It frequently occurs in the nape of the neck and usually Skin Grafting starts as an infection of a hair follicle or a group of hair follicles. Skin grafting is of two main types: split-thickness grafting and full-thickness grafting. In a split-thickness graft, the greater Sebaceous Cyst part of the epidermis, including the tips of the dermal papillae, A sebaceous cyst is caused by obstruction of the mouth of a is removed from the donor site and placed on the recipient site. sebaceous duct and may be caused by damage from a comb or This leaves at the donor site for repair purposes the epidermal by infection. It occurs most frequently on the scalp. cells on the sides of the dermal papillae and the cells of the hair follicles and sweat glands. Shock A full-thickness skin graft includes both the epidermis and the A patient who is in a state of shock is pale and exhibits goose- dermis and, to survive, requires rapid establishment of a new cir- flesh as a result of overactivity of the sympathetic system, which culation within it at the recipient site. The donor site is usually causes vasoconstriction of the dermal arterioles and contraction covered with a split-thickness graft. In certain circumstances, the of the arrector pili muscles. full-thickness graft is made in the form of a pedicle graft, in which a flap of full-thickness skin is turned and stitched in position at the Skin Burns recipient site, leaving the base of the flap with its blood supply The depth of a burn determines the method and rate of healing. intact at the donor site. Later, when the new blood supply to the A partial-skin-thickness burn heals from the cells of the hair graft has been established, the base of the graft is cut across.  Basic Anatomy 7 biceps musculocutaneous nerve median nerve cephalic vein humerus brachial artery brachialis ulnar nerve lateral intermuscular medial septum intermuscular septum coracobrachialis radial nerve deep fascia triceps superficial fascia extensor tendons skin and their synovial FIGURE 1.6 Section through the middle of the right arm sheaths showing the arrangement of the superficial and deep fascia. Note how the fibrous septa extend between groups of muscles, dividing the arm into fascial compartments. extensor retinaculum Fasciae The fasciae of the body can be divided into two types— FIGURE 1.7 Extensor retinaculum on the posterior surface superficial and deep—and lie between the skin and the of the wrist holding the underlying tendons of the extensor underlying muscles and bones. muscles in position. The superficial fascia, or subcutaneous tissue, is a mix- ture of loose areolar and adipose tissue that unites the ­dermis of the skin to the underlying deep fascia (Fig. 1.6). Muscle In the scalp, the back of the neck, the palms of the hands, The three types of muscle are skeletal, smooth, and cardiac. and the soles of the feet, it contains numerous bundles of collagen fibers that hold the skin firmly to the deeper struc- Skeletal Muscle tures. In the eyelids, auricle of the ear, penis and scrotum, Skeletal muscles produce the movements of the skeleton; and clitoris, it is devoid of adipose tissue. they are sometimes called voluntary muscles and are made The deep fascia is a membranous layer of connective tis- up of striped muscle fibers. A skeletal muscle has two or sue that invests the muscles and other deep structures (see Fig. more attachments. The attachment that moves the least is 1.6). In the neck, it forms well-defined layers that may play an referred to as the origin, and the one that moves the most, important role in determining the path taken by pathogenic the insertion (Fig. 1.8). Under varying circumstances, the organisms during the spread of infection. In the thorax and abdomen, it is merely a thin film of areolar tissue covering the muscles and aponeuroses. In the limbs, it forms a defi- nite sheath around the muscles and other structures, holding them in place. Fibrous septa extend from the deep surface of the membrane, between the groups of muscles, and in many places divide the interior of the limbs into compartments (see Fig. 1.6). In the region of joints, the deep fascia may be consid- origin erably thickened to form restraining bands called retinacula (Fig. 1.7). Their function is to hold underlying tendons in posi- tion or to serve as pulleys around which the tendons may move. belly C L I N I C A L N O T E S gastrocnemius Fasciae and Infection A knowledge of the arrangement of the deep fasciae often helps explain the path taken by an infection when it spreads insertion from its primary site. In the neck, for example, the various fascial planes explain how infection can extend from the region of the floor of the mouth to the larynx. FIGURE 1.8 Origin, insertion, and belly of the gastrocnemius muscle. 8 Chapter 1 Introduction that muscles whose fibers run parallel to the line of pull will bring about a greater degree of movement compared with those whose fibers run obliquely. Examples of muscles with parallel fiber arrangements (see Fig. 1.10) are the ster- nocleidomastoid, the rectus abdominis, and the sartorius. Muscles whose fibers run obliquely to the line of pull are referred to as pennate muscles (they resemble a feather) (see Fig. 1.10). A unipennate muscle is one in which the tendon lies along one side of the muscle and the muscle fibers pass obliquely to it (e.g., extensor digitorum lon- gus). A bipennate muscle is one in which the tendon lies in the center of the muscle and the muscle fibers pass to it from two sides (e.g., rectus femoris). A multipennate mus- cle may be arranged as a series of bipennate muscles lying alongside one another (e.g., acromial fibers of the deltoid) or may have the tendon lying within its center and the mus- cle fibers passing to it from all sides, converging as they go B (e.g., tibialis anterior). For a given volume of muscle substance, pennate mus- A cles have many more fibers compared to muscles with par- allel fiber arrangements and are therefore more powerful; in other words, range of movement has been sacrificed for strength. Skeletal Muscle Action common tendon for the insertion external oblique aponeurosis of the gastrocnemius and All movements are the result of the coordinated action of soleus muscles many muscles. However, to understand a muscle’s action, it is necessary to study it individually. A muscle may work in the following four ways: Prime mover: A muscle is a prime mover when it is the chief muscle or member of a chief group of muscles responsible for a particular movement. For example, the quadriceps femoris is a prime mover in the movement of extending the knee joint (Fig. 1.11). Antagonist: Any muscle that opposes the action of the prime mover is an antagonist. For example, the biceps C femoris opposes the action of the quadriceps femoris when the knee joint is extended (see Fig. 1.11). Before a raphe of mylohyoid muscles prime mover can contract, the antagonist muscle must FIGURE 1.9 Examples of (A) a tendon, (B) an aponeurosis, be equally relaxed; this is brought about by nervous and (C) a raphe. reflex inhibition. Fixator: A fixator contracts isometrically (i.e., contraction increases the tone but does not in itself produce move- degree of mobility of the attachments may be reversed; ment) to stabilize the origin of the prime mover so that it therefore, the terms origin and insertion are interchangeable. can act efficiently. For example, the muscles attaching the The fleshy part of the muscle is referred to as its belly shoulder girdle to the trunk contract as fixators to allow (see Fig. 1.8). The ends of a muscle are attached to bones, the deltoid to act on the shoulder joint (see Fig. 1.11). cartilage, or ligaments by cords of fibrous tissue called Synergist: In many locations in the body, the prime mover tendons (Fig. 1.9). Occasionally, flattened muscles are muscle crosses several joints before it reaches the joint at attached by a thin but strong sheet of fibrous tissue called which its main action takes place. To prevent unwanted an aponeurosis (see Fig. 1.9). A raphe is an interdigita- movements in an intermediate joint, groups of muscles tion of the tendinous ends of fibers of flat muscles (see called synergists contract and stabilize the intermediate Fig. 1.9). joints. For example, the flexor and extensor muscles of Internal Structure of Skeletal Muscle the carpus contract to fix the wrist joint, and this allows the long flexor and the extensor muscles of the fingers to The muscle fibers are bound together with delicate areolar work efficiently (see Fig. 1.11). tissue, which is condensed on the surface to form a fibrous envelope, the epimysium. The individual fibers of a muscle These terms are applied to the action of a particular mus- are arranged either parallel or oblique to the long axis of the cle during a particular movement; many muscles can act muscle (Fig. 1.10). Because a muscle shortens by one third as a prime mover, an antagonist, a fixator, or a synergist, to one half its resting length when it contracts, it follows depending on the movement to be accomplished.  Basic Anatomy 9 rhomboid quadrilateral strap strap with fusiform two bellies two headed triangular tendinous intersections unipennate bipennate multipennate relaxed contracted FIGURE 1.10 Different forms of the internal structure of skeletal muscle. A relaxed and a contracted muscle are also shown; note how the muscle fibers, on contraction, shorten by one third to one half of their resting length. Note also how the mus- cle swells. Muscles can even contract paradoxically, for example, often near the margin; the place of entrance is known when the biceps brachii, a flexor of the elbow joint, con- as the motor point. This arrangement allows the mus- tracts and controls the rate of extension of the elbow when cle to move with minimum interference with the nerve the triceps brachii contracts. trunk. Nerve Supply of Skeletal Muscle Naming of Skeletal Muscles The nerve trunk to a muscle is a mixed nerve, about Individual muscles are named according to their shape, 60% is motor and 40% is sensory, and it also contains size, number of heads or bellies, position, depth, attach- some sympathetic autonomic fibers. The nerve enters ments, or actions. Some examples of muscle names are the muscle at about the midpoint on its deep surface, shown in Table 1.1. C L I N I C A L N O T E S Muscle Tone knowledge is it possible to understand the normal and abnormal Determination of the tone of a muscle is an important clinical actions of individual muscles or muscle groups. How can one examination. If a muscle is flaccid, then either the afferent, the even attempt to analyze, for example, the abnormal gait of a efferent, or both neurons involved in the reflex arc necessary for patient without this information? the production of muscle tone have been interrupted. For example, Muscle Shape and Form if the nerve trunk to a muscle is severed, both neurons will have been interrupted. If poliomyelitis has involved the motor anterior The general shape and form of muscles should also be noted, horn cells at a level in the spinal cord that innervates the muscle, since a paralyzed muscle or one that is not used (such as the efferent motor neurons will not function. If, conversely, the occurs when a limb is immobilized in a cast) quickly atrophies muscle is found to be hypertonic, the possibility exists of a lesion and changes shape. In the case of the limbs, it is always worth involving higher motor neurons in the spinal cord or brain. remembering that a muscle on the opposite side of the body can be used for comparison. Muscle Attachments The importance of knowing the main attachments of all the major muscles of the body need not be emphasized. Only with such 10 Chapter 1 Introduction quadriceps quadriceps biceps femoris biceps femoris A B rhomboid minor rhomboid major deltoid serratus anterior serratus anterior scapula C rhomboid extensor digitorum extensor carpi radialis flexor digitorum flexor carpi D profundus radialis FIGURE 1.11 Different types of muscle action. A. Quadriceps femoris extending the knee as a prime mover, and biceps femo- ris acting as an antagonist. B. Biceps femoris flexing the knee as a prime mover, and quadriceps acting as an antagonist. C. Muscles around shoulder girdle fixing the scapula so that movement of abduction can take place at the shoulder joint. D. Flexor and extensor muscles of the carpus acting as synergists and stabilizing the carpus so that long flexor and extensor tendons can flex and extend the fingers. Smooth Muscle with one another. Their contraction is slow and sustained Smooth muscle consists of long, spindle-shaped cells closely and brings about expulsion of the contents of the organs. arranged in bundles or sheets. In the tubes of the body, it In the walls of the blood vessels, the smooth muscle fibers provides the motive power for propelling the contents are arranged circularly and serve to modify the caliber of through the lumen. In the digestive system, it also causes the lumen. the ingested food to be thoroughly mixed with the digestive Depending on the organ, smooth muscle fibers may juices. A wave of contraction of the circularly arranged fib- be made to contract by local stretching of the fibers, by ers passes along the tube, milking the contents onward. By nerve impulses from autonomic nerves, or by hormonal their contraction, the longitudinal fibers pull the wall of the ­stimulation. tube proximally over the contents. This method of propul- sion is referred to as peristalsis. Cardiac Muscle In storage organs such as the urinary bladder and the Cardiac muscle consists of striated muscle fibers that uterus, the fibers are irregularly arranged and interlaced branch and unite with each other. It forms the ­myocardium  Basic Anatomy 11 TA B L E 1. 1 Naming of Skeletal Musclesa Number of Heads or Name Shape Size Bellies Position Depth Attachments Actions Deltoid Triangular Teres Round Rectus Straight Major Large Latissimus Broadest Longissimus Longest Biceps Two heads Quadriceps Four heads Digastric Two bellies Pectoralis Of the chest Supraspinatus Above spine of scapula Brachii Of the arm Profundus Deep Superficialis Superficial Externus External Sternocleidomastoid From sternum and clavicle to mastoid process Coracobrachialis From coracoid ­process to arm Extensor Extend Flexor Flex Constrictor Constrict a These names are commonly used in combination, for example, flexor pollicis longus (long flexor of the thumb). of the heart. Its fibers tend to be arranged in whorls and Joints spirals, and they have the property of spontaneous and rhythmic contraction. Specialized cardiac muscle fibers A site where two or more bones come together, whether form the conducting system of the heart. or not movement occurs between them, is called a joint. Cardiac muscle is supplied by autonomic nerve fibers Joints are classified according to the tissues that lie between that terminate in the nodes of the conducting system and the bones: fibrous joints, cartilaginous joints, and synovial in the myocardium. joints. Fibrous Joints The articulating surfaces of the bones are joined by fibrous C L I N I C A L N O T E S tissue (Fig. 1.12), and thus very little movement is possible. The sutures of the vault of the skull and the inferior tibi- ofibular joints are examples of fibrous joints. Necrosis of Cardiac Muscle The cardiac muscle receives its blood supply from the coro- Cartilaginous Joints nary arteries. A sudden block of one of the large branches of a Cartilaginous joints can be divided into two types: primary coronary artery will inevitably lead to necrosis of the cardiac and secondary. A primary cartilaginous joint is one in muscle and often to the death of the patient. which the bones are united by a plate or a bar of hyaline cartilage. Thus, the union between the epiphysis and the 12 Chapter 1 Introduction periosteum suture skull bone fibrous joint skull skull bone periosteum A posterior longitudinal ligament ligamentum flavum fibrocartilaginous intervertebral disc cartilaginous joint anterior longitudinal ligament vertebral column interspinous supraspinous B ligament ligament hip bone hyaline articular cartilage synovial joint fibrous capsule fatty pad hip joint ligamentum teres C femur synovial membrane FIGURE 1.12 Examples of three types of joints. A. Fibrous joint (coronal suture of skull). B. Cartilaginous joint (joint between two lumbar vertebral bodies). C. Synovial joint (hip joint). diaphysis of a growing bone and that between the 1st rib Synovial Joints and the manubrium sterni are examples of such a joint. No The articular surfaces of the bones are covered by a thin layer movement is possible. of hyaline cartilage separated by a joint cavity (see Fig. 1.12). A secondary cartilaginous joint is one in which the This arrangement permits a great degree of freedom of bones are united by a plate of fibrocartilage and the articu- movement. The cavity of the joint is lined by synovial lar surfaces of the bones are covered by a thin layer of hya- membrane, which extends from the margins of one articu- line cartilage. Examples are the joints between the vertebral lar surface to those of the other. The synovial membrane bodies (see Fig. 1.12) and the symphysis pubis. A small is protected on the outside by a tough fibrous membrane amount of movement is possible.  Basic Anatomy 13 referred to as the capsule of the joint. The articular surfaces Ellipsoid joints: In ellipsoid joints, an elliptical ­convex are lubricated by a viscous fluid called synovial fluid, which articular surface fits into an elliptical c­ oncave ­articular is produced by the synovial membrane. In certain synovial surface. The movements of flexion, extension, abduction, joints, for example, in the knee joint, discs or wedges of and adduction can take place, but rotation is impossible. fibrocartilage are interposed between the articular surfaces The wrist joint is a good e­ xample (see Fig. 1.14). of the bones. These are referred to as articular discs. Saddle joints: In saddle joints, the articular surfaces are Fatty pads are found in some synovial joints lying reciprocally concavoconvex and resemble a saddle on between the synovial membrane and the fibrous capsule a horse’s back. These joints permit flexion, extension, or bone. Examples are found in the hip (see Fig. 1.12) and abduction, adduction, and rotation. The best example knee joints. of this type of joint is the carpometacarpal joint of the The degree of movement in a synovial joint is limited thumb (see Fig. 1.14). by the shape of the bones participating in the joint, the Ball-and-socket joints: In ball-and-socket joints, a ball- coming together of adjacent anatomic structures (e.g., the shaped head of one bone fits into a socketlike concavity thigh against the anterior abdominal wall on flexing the of another. This arrangement permits free movements, hip joint), and the presence of fibrous ligaments uniting including flexion, extension, abduction, adduction, the bones. Most ligaments lie outside the joint capsule, but medial rotation, lateral rotation, and circumduction. in the knee some important ligaments, the cruciate liga- The shoulder and hip joints are good examples of this ments, lie within the capsule (Fig. 1.13). type of joint (see Fig. 1.14). Synovial joints can be classified according to the arrange- ment of the articular surfaces and the types of movement Stability of Joints that are possible. The stability of a joint depends on three main factors: the shape, size, and arrangement of the articular surfaces; the Plane joints: In plane joints, the apposed articular sur- ligaments; and the tone of the muscles around the joint. faces are flat or almost flat, and this permits the bones to slide on one another. Examples of these joints are the Articular Surfaces sternoclavicular and acromioclavicular joints (Fig. 1.14). The ball-and-socket arrangement of the hip joint (see Hinge joints: Hinge joints resemble the hinge on a door, Fig. 1.13) and the mortise arrangement of the ankle joint so that flexion and extension movements are possible. are good examples of how bone shape plays an important Examples of these joints are the elbow, knee, and ankle role in joint stability. Other examples of joints, however, joints (see Fig. 1.14). in which the shape of the bones contributes little or noth- Pivot joints: In pivot joints, a central bony pivot is sur- ing to the stability include the acromioclavicular joint, the rounded by a bony–ligamentous ring (see Fig. 1.14), and calcaneocuboid joint, and the knee joint. rotation is the only movement possible. The atlantoaxial and superior radioulnar joints are good examples. Ligaments Condyloid joints: Condyloid joints have two distinct Fibrous ligaments prevent excessive movement in a joint convex surfaces that articulate with two concave sur- (see Fig. 1.13), but if the stress is continued for an excessively faces. The movements of flexion, extension, abduc- long period, then fibrous ligaments stretch. For example, the tion, and adduction are possible together with a small ligaments of the joints between the bones forming the arches amount of rotation. The metacarpophalangeal joints or of the feet will not by themselves support the weight of the knuckle joints are good examples (see Fig. 1.14). body. Should the tone of the m ­ uscles that normally support hemispherical cup-shaped peroneus longus muscle holding head of femur acetabulum cruciate up lateral longitudinal arch ligaments of right foot peroneus ligament medial collateral ligament hip joint knee joint arch of foot A B C FIGURE 1.13 The three main factors responsible for stabilizing a joint. A. Shape of articular surfaces. B. Ligaments. C. Muscle tone. 14 Chapter 1 Introduction clavicle acromioclavicular joint humerus sternum acromion elbow joint sternoclavicular joint radius scapula B A ulna metacarpal metacarpal phalanx phalanx atlas metacarpal axis D phalanx C radius ulna lunate metacarpal of thumb scaphoid hip bone triquetral femur trapezium E F G FIGURE 1.14 Examples of different types of synovial joints. A. Plane joints (sternoclavicular and acromioclavicular joints). B. Hinge joint (elbow joint). C. Pivot joint (atlantoaxial joint). D. Condyloid joint (metacarpophalangeal joint). E. Ellipsoid joint (wrist joint). F. Saddle joint (carpometacarpal joint of the thumb). G. Ball-and-socket joint (hip joint). the arches become impaired by fatigue, then the ligaments Muscle Tone will stretch and the arches will collapse, producing flat feet. In most joints, muscle tone is the major factor controlling Elastic ligaments, conversely, return to their original length stability. For example, the muscle tone of the short muscles after stretching. The elastic ligaments of the auditory ossicles around the shoulder joint keeps the hemispherical head of play an active part in supporting the joints and assisting in the the humerus in the shallow glenoid cavity of the scapula. return of the bones to their original position after movement. Without the action of these muscles, very little force would  Basic Anatomy 15 be required to dislocate this joint. The knee joint is very Nerve Supply of Joints unstable without the tonic activity of the quadriceps fem- The capsule and ligaments receive an abundant sensory oris muscle. The joints between the small bones forming nerve supply. A sensory nerve supplying a joint also sup- the arches of the feet are largely supported by the tone of plies the muscles moving the joint and the skin overlying the muscles of the leg, whose tendons are inserted into the the insertions of these muscles, a fact that has been codified bones of the feet (see Fig. 1.13). as Hilton’s law. C L I N I C A L N O T E S Examination of Joints In certain diseases of the nervous system (e.g., syringomyelia), When examining a patient, the clinician should assess the nor- the sensation of pain in a joint is lost. This means that the warn- mal range of movement of all joints. When the bones of a joint ing sensations of pain felt when a joint moves beyond the nor- are no longer in their normal anatomic relationship with one mal range of movement are not experienced. This phenomenon another, then the joint is said to be dislocated. Some joints are results in the destruction of the joint. particularly susceptible to dislocation because of lack of sup- The knowledge of the classification of joints is of great value port by ligaments, the poor shape of the articular surfaces, or because, for example, certain diseases affect only certain types the absence of adequate muscular support. The shoulder joint, of joints. Gonococcal arthritis affects large synovial joints such temporomandibular joint, and acromioclavicular joints are good as the ankle, elbow, or wrist, whereas tuberculous arthritis also examples. Dislocation of the hip is usually congenital, being affects synovial joints and may start in the synovial membrane caused by inadequate development of the socket that normally or in the bone. holds the head of the femur firmly in position. Remember that more than one joint may receive the same The presence of cartilaginous discs within joints, especially nerve supply. For example, both the hip and knee joints are sup- weightbearing joints, as in the case of the knee, makes them par- plied by the obturator nerve. Thus, a patient with disease limited ticularly susceptible to injury in sports. During a rapid movement, to one of these joints may experience pain in both. the disc loses its normal relationship to the bones and becomes crushed between the weightbearing surfaces. Ligaments Bursae A ligament is a cord or band of connective tissue uniting A bursa is a lubricating device consisting of a closed fibrous two structures. Commonly found in association with joints, sac lined with a delicate smooth membrane. Its walls are ligaments are of two types. Most are composed of dense separated by a film of viscous fluid. Bursae are found wher- bundles of collagen fibers and are unstretchable under nor- ever tendons rub against bones, ligaments, or other ten- mal conditions (e.g., the iliofemoral ligament of the hip dons. They are commonly found close to joints where the joint and the collateral ligaments of the elbow joint). The skin rubs against underlying bony structures, for example, second type is composed largely of elastic tissues and can the prepatellar bursa (Fig. 1.15). Occasionally, the cavity of therefore regain its original length after stretching (e.g., the a bursa communicates with the cavity of a synovial joint. ligamentum flavum of the vertebral column and the calca- For example, the suprapatellar bursa communicates with neonavicular ligament of the foot). the knee joint (see Fig. 1.15) and the subscapularis bursa communicates with the shoulder joint. C L I N I C A L N O T E S Synovial Sheath A synovial sheath is a tubular bursa that surrounds a tendon. The tendon invaginates the bursa from one side so that the Damage to Ligaments tendon becomes suspended within the bursa by a mesoten- Joint ligaments are very prone to excessive stretching and don (see Fig. 1.15). The mesotendon enables blood vessels to even tearing and rupture. If possible, the apposing damaged enter the tendon along its course. In certain situations, when surfaces of the ligament are brought together by positioning the range of movement is extensive, the mesotendon disap- and immobilizing the joint. In severe injuries, surgical approxi- pears or remains in the form of narrow threads, the vincula mation of the cut ends may be required. The blood clot at the (e.g., the long flexor tendons of the fingers and toes). damaged site is invaded by blood vessels and fibroblasts. The Synovial sheaths occur where tendons pass under liga- fibroblasts lay down new collagen and elastic fibers, which ments and retinacula and through osseofibrous tunnels. become oriented along the lines of mechanical stress. Their function is to reduce friction between the tendon and its surrounding structures. 16 Chapter 1 Introduction Blood Vessels C L I N I C A L N O T E S Blood vessels are of three types: arteries, veins, and capil- laries (Fig. 1.16). Trauma and Infection of Bursae and Synovial Arteries transport blood from the heart and distrib- Sheaths ute it to the various tissues of the body by means of their Bursae and synovial sheaths are commonly the site of trau- branches (Figs. 1.16 and 1.17). The smallest arteries, matic or infectious disease. For example, the extensor tendon