Bones and Cartilages PDF

# Bones and Cartilages ##Skeleton - The hard, supporting framework of the body is known as the **skeleton**. - It serves as a scaffold that supports organs, anchors muscles, and protects vital organs like the brain, spinal cord, heart, and lungs. - It consists mainly of **bones** which are composed...

# Bones and Cartilages ##Skeleton - The hard, supporting framework of the body is known as the **skeleton**. - It serves as a scaffold that supports organs, anchors muscles, and protects vital organs like the brain, spinal cord, heart, and lungs. - It consists mainly of **bones** which are composed of mineralized tissue and, therefore, are hard and strong. - A small part of the human skeleton consists of **cartilages** that are composed of semirigid connective tissue which is strong and flexible but not hard because it is not mineralized. ## Bones - The bones (Latin synonym “os”) are made up of osseous tissue, which has the strength of cast iron and lightness of pinewood. - Bone is a variety of connective tissue and, like other connective tissues, it consists of cells (mainly **osteocytes**) and intercellular substance or **matrix** of the bone. - The matrix contains **collagen fibers** and ground substance. - The ground substance of the osseous tissue is impregnated with mineral salts (mainly **calcium phosphate**) which impart hardness and rigidity to this tissue. ## Functions of Bones ### Mechanical Functions 1. The bones make up the skeleton and provide the rigid framework that supports the body. 2. They protect certain vital organs like brain, spinal cord, heart, lungs, and organs of the pelvis by forming sturdy walls of the body cavity. 3. The bones also make body movements possible by providing attachment sites for muscles and by acting as levers for joints. ### Physiological Functions 1. Bone is the main storehouse and supplier of **calcium**, **phosphate** and **magnesium salts**. 2. The bones contain marrow which is the site of blood cell formation. ## Types of Bone Tissue 1. **Compact bone tissue** 2. **Spongy bone tissue** - The **compact bone tissue** is very hard and dense. - It appears solid to the naked eye but microscopic examination reveals that it contains cylinders of calcified bone called **Haversian systems** or **osteons**. - In the center of each osteon is present a longitudinal channel, called **Haversian canal**, which contains blood vessels, lymph vessels and nerves. - The structure of osteons provides great strength needed to resist the stresses which the bones are subjected every day. - The **spongy bone tissue** presents an interlaced pattern of tissue consisting of an irregular meshwork of intercommunicating bony **trabeculae**. - The spaces of the meshwork contain **red bone marrow**. - It is also called **cancellous bone**. - The spongy bone is present inside most bones where it is found to be surrounded by an outer shell of compact bone. - This structure provides great strength with least weight. ## Regional Distribution of Bones - Depending on their location, the bones are divided into two main groups: 1. **Axial bones** 2. **Appendicular bones** ### Axial Bones - Axial bones lie in the region of head, neck, and trunk. - Their distribution is as under: - **Skull:** 22 bones - **Ear ossicles:** 6 bones - **Hyoid:** 1 bone - **Vertebral column:** 26 bones - **Ribs:** 24 bones - **Sternum:** 1 bone - **Total** **80 bones** ### Appendicular Bones - Appendicular bones belong to the extremities (limbs). - Their distribution is as follows: - **Upper extremities:** 64 bones - **Lower extremities:** 62 bones - **Total** **126 bones** - The above-given figures show that the total number of bones in an adult is 206 (80 axial + 126 appendicular bones). - However, this is only an approximation because a number of accessory bones may also be present. ## Classification of Bones ### Developmental Classification - According to the mode of their development during the embryonic life, the bones are categorized into three main types: 1. **Membrane bones** 2. **Cartilaginous bones** 3. **Membranocartilaginous bones**. #### Membrane Bones - These bones are formed by the **intramembranous method of osteogenesis**. - These bones develop in membranous sheets formed by condensation of the mesenchyme. - Examples of the membrane bones are: - Flat bones of the skull vault, and - Bones of the face (maxilla, zygomatic, and nasal bones, etc.). #### Cartilaginous Bones - These bones develop by the **intracartilaginous method of osteogenesis**. - These bones develop in preformed models of hyaline cartilage. - Examples of the cartilaginous bones are ribs, and vertebrae, and bones of the limbs. #### Membranocartilaginous Bones - These bones develop partly in membrane and partly in cartilage. - The best examples are the occipital, temporal and sphenoid bones of the skull. - The mandible and clavicle are also included in this category of bones. ### Regional Classification - As explained earlier, the bones of the body are divided into main groups upon the basis of their regional location: - **Bones of the Axial Skeleton** - **Bones of the Appendicular Skeleton** #### Bones of the Axial Skeleton - This group consists of the bones of the head and trunk which include the skull bones, ossicles of the middle ear, the rib cage, sternum, and the vertebral column. #### Bones of the Appendicular Skeleton - This group includes the bones of the upper and lower limbs. ### Classification According to Size and Shape - On the basis of the size, shape and some other structural features, the bones are classified into the following types: - **Long bones** - **Short bones** - **Flat bones** - **Irregular bones** - **Sesamoid bones** - **Accessory bones** #### Long Bones - A long bone consists of a cylindrical shaft and two expanded ends, which are smooth and articular. - The long bones of the body are subclassified into two types: - **Typical long bones** - **Miniature long bones**. ##### Typical Long Bones - These bones possess considerable length. - They include the humerus, radius and ulna of the upper limb and the femur, tibia and fibula of the lower limb. ##### Miniature Long Bones - The principal feature of these long bones is their short length. - They include the metacarpals and phalanges of hand, and the metatarsals and phalanges of foot. #### Short Bones - These bones have nearly the same dimension in length, width, and thickness but are usually shaped irregularly. - They occur only in the wrists (carpal bones) and ankles (tarsal bones), where only limited movement is required. - Short bones also consist of spongy bone with an outer shell of compact bone. - Most of their surface area is occupied by cartilage covered articular surfaces, where joints are formed with adjacent bones. - There are some nonarticular areas for the entry of blood vessels and for the attachment of muscles and ligaments. #### Flat Bones - They usually occur as thin and curved plates. - They serve as protective covering and provide broad surfaces for muscle attachment. - They include the ribs, sternum, scapulae, and bones of skull vault. - A flat bone consists of two layers of compact bone, between which is sandwiched a thin layer of spongy bone. - In the flat bones of the skull, the outer and inner layers of the compact bone are called **tables**, while the spongy bone between them is called **diploe** which contains red bone marrow in life. #### Irregular Bones - These bones do not fit into any other category. - They have an irregular shape with several processes. - They are composed of spongy bone (containing red marrow) covered by compact bone. - This variety of bones includes hip bones, vertebrae bones of base of skull (sphenoid, temporal, mastoid, etc.) and bones of the face (maxilla, zygomatic, etc.). - Some bones of the skull contain air-filled cavities called **air sinuses** or **air cells**. - These bones are called **pneumatic bones**. - Examples are maxilla, ethmoid and mastoid bones. - The air sinuses in the skull bones are clinically important as they are favourite sites of infection. #### Sesamoid Bones - A sesamoid bone is a bony nodule embedded within a tendon or muscle. - Sesamoid bones are so named because the smallest ones resemble the seeds of the sesame plant. - Their number is not fixed because they occur regularly in some locations and with variable frequency in others. #### Accessory Bones - These bones are not always present. - They are further classified into the following subtypes: - **Supernumerary Bones** - **Wormian Bones** - **Heterotropic Bones** ##### Supernumerary Bones - Sometimes an extra secondary center of ossification appears in a bone, which does not fuse with the main bone mass and gives rise to a separate small bone. - These accessory bones formed in this manner are known as supernumerary bones. - They are commonly found in the foot and, in a radiograph of the foot, these bones may be mistaken for bone chips resulting from injury. - Examples of the supernumerary bones are: - **os vesalianum** which may be present adjacent to the tuberosity of the 5th metatarsal bone, and - **os trigonum** which sometimes occurs adjacent to the lateral tubercle on the posterior surface of the talus. ##### Wormian Bones - Accessory bones commonly occur in the cranium as small bony islands within the sutural joints between the bones of the skull vault. - Such bones are called **Wormian** bones or **sutural bones**. - They are most commonly seen in the course of the lambdoid suture of skull. ##### Heterotropic Bones - Sometimes accessory bones develop in muscles or other soft tissues; such bones are called **heterotropic bones**. - Best examples are the **rider's bones**, which develop in the horse riders in the adductor muscles of the thigh. ## Markings (Impressions) on a Bone - The surfaces of dried bones present various bony markings in the form of roughened areas, processes, depressions, holes, and grooves. - These markings serve a variety of functions including muscle and ligament attachments, passage of blood vessels and nerves, and articulations with other bones. ### Articular Surfaces - These include those parts of the bones that come into contact with other bones to form joints. - They may take following forms. - **Head** - **Capitulum** - **Trochlea** - **Condyle** - **Facet** - **Fossa** ##### Head - A large rounded articular surface, often set off from the body of the bone by a neck is known as head of the bone. - For example, head of femur. ##### Capitulum - A small, rounded articular eminence is called a capitulum. - Best example is the capitulum of the humerus. ##### Trochlea - A pulley-shaped articular surface is known as a trochlea. - For example, trochlea of the humerus. ##### Condyle - A rounded protuberance at the end of a bone is known as a condyle. - The condyles usually occur in pairs. - For example, the medial and lateral condyles of the femur. ##### Facet - A small, smooth, flat articular surface is referred to as a facet. - For example, the articular facet on the acromion of scapula. ##### Fossa - A hollow depressed region on a bone is called a fossa. - For example, olecranon fossa of the humerus. ### Nonarticular Surfaces - These surfaces generally serve for the attachment of muscles and ligaments. - They may be of the following types: - **Process** - **Trochanter** - **Tubercle** - **Tuberosity** - **Malleolus** - **Spine** - **Hamulus** - **Crest** - **Line** - **Fovea** - **Notch** - **Epicondyle** ##### Process - A broad term that denotes a projection from a bone. - The bone processes are generally named according to their shape. - For example, the coracoid process of scapula is so named because it resembles the beak of a crow. - The rounded projection from the temporal bone of the skull is titled **mastoid process** (breast-shaped). - The long, pointed projection from the temporal bone is named **styloid process**, meaning shaped like a stylus. ##### Trochanter - A relatively large, blunt prominence on a bone is known as a trochanter. - For example, the grater and lesser trochanters of the femur. ##### Tubercle - A small round projection or excrescence from a bone is called a tubercle. - For example, the greater and lesser tubercles of the humerus. ##### Tuberosity - A rough projection or protuberance on a bone is called tuberosity. - For example, the deltoid tuberosity of the humerus and the gluteal tuberosity of the femur. ##### Malleolus - A bony projection having the shape of a hammerhead is called a malleolus. - For example, medial malleolus of the tibia and lateral malleolus of the fibula. ##### Spine - Generally, a short, pointed projection from a bone is known as a spine. - For example, the anterior nasal spine which is a small, pointed projection form the maxilla. - A prominent triangular plate of bone on the dorsal surface of the scapula is also called "spine" of scapula. ##### Hamulus - A hook-like projection from a bone is called a hamulus. - For example, the pterygoid hamulus which is a hook-like process at the lower and of the medial pterygoid plate of the sphenoid bone of the skull. ##### Crest - A prominent, usually roughened ridge on the surface of a bone is called crest. - For example, intertrochanteric crest of the femur. ##### Line - A less prominent ridge on a bone surface is known as a line. - For example, intertrochanteric line of the femur. ##### Fovea - A small, shallow depression on the surface of a bone is called fovea. - For example, fovea capitis which is a small pit on the surface of the head of femur. ##### Notch - An indentation in the edge of a bone is known as a notch. - For example, the suprascapular notch on the superior border of the scapula. ##### Epicondyle - A projection on or above a condyle is called an epicondyle. - For example, the lateral and medial epicondyles of the humerus and the lateral and medial epicondyles of the femur. ### Grooves and Holes - These usually serve for passage of vessels and nerves and are categorized as under: - **Fissure** - **Foramen** - **Meatus** - **Sulcus** - **Hiatus** ##### Fissure - A narrow cleft or deep groove in a bone (or between two adjacent bones) is referred to as a fissure. ##### Foramen - A hole to allow the vessels or nerves to pass into or through a bone is called a foramen. - For example, the nutrient foramen of the humerus. ##### Meatus - A short canal or passage within a bone is known as a meatus. - For example, the external auditory meatus in the temporal bone of the skull. ##### Sulcus - A shallow groove or furrow on the surface of a bone is called a sulcus. - For example, the intertubercular sulcus on the humerus. ##### Hiatus - A slit-like gap or cleft in a bone is known as a hiatus. ## Gross Anatomy of an Adult Typical Long Bone - A typical long bone consists of a roughly cylindrical shaft and two expanded ends. ### The Shaft of a Long Bone - The shaft, also called **diaphysis**, is like a hollow cylinder. - The central cavity of the cylindrical shaft is called **medullary cavity**. - The walls of the cylinder are composed mainly of compact bone, called **cortex** of the shaft. - The outer surface of the cortex is covered by a layer of connective tissue called **periosteum**. - The periosteum is a thick fibrous membrane covering the surface of the bone. - It contains blood capillaries, lymphatic vessels and nerves. - It also lodges **osteoprogenitor cells**, which have the potential to transform into the bone-forming cells called **osteoblasts**. - The cortex is made up of compact bone, so that the shaft is strong enough to withstand all possible mechanical strains. - The medullary cavity is a hollow space that runs through the length of the diaphysis. - In living bone, this cavity contains **yellow bone marrow**, which is composed of blood vessels and fat cells. - The medullary cavity is lined by **endosteum**, which is a thin layer of connective tissue containing osteoprogenitor cells. - During bone development, the shaft (diaphysis) develops from the primary center of ossification (described later). ### The Ends of a Long Bone - Each end of a long bone consists mainly of spongy bone which is covered by a thin shell of compact bone. - In life, the spaces between the thin plates and spicules of the spongy bone are occupied by the **red bone marrow**. - The bone ends usually, but not always, bear smooth **articular surfaces** to participate in the formation of joints with the other bones. - In life, the articular surfaces are covered by a layer of hyaline cartilage which is called **articular cartilage**. - During bone development, the bone ends develop from secondary centers of ossification. ## Epiphyses - Any part of a bone which is formed from a secondary center of ossification is known as an **epiphysis**. - In a developing bone, the epiphyses are separated from the main body of the bone by a layer of cartilage but unite with the main bone mass when ossification of the bone is complete. ### Types of Epiphyses - Generally four types of epiphyses are recognized in the human body: **pressure epiphyses**, **traction epiphyses**, **atavistic epiphyses**, and **aberrant epiphyses**. #### Pressure Epiphyses - These epiphyses are present at those bone ends which are either involved in transmitting the weight of the human body or at those bone regions which are under pressure during movement or locomotion. - The pressure epiphyses are always articular and take part in the formation of joints. - Typical examples of pressure epiphyses are head of the femur and head of the humerus. #### Traction Epiphyses - These epiphyses are non-articular and do not form joints. - The main function of the traction epiphyses is to provide attachment sites for the tendons and ligaments. - The traction epiphyses ossify later than the pressure epiphyses. - Examples of the traction epiphyses are the grater and lesser trochanters of femur and the greater and lesser tubercles of the humerus. #### Atavistic Epiphyses - These epiphyses represent those bones which were phylogenetically independent (e.g., in the four-legged animals) but have become fused with other bones in the humans. - A typical example of atavistic epiphyses is the coracoid process of scapula. #### Aberrant Epiphyses - As the name indicates, these epiphyses are deviations from the normal and are not always present. - Examples are provided by the presence of aberrant epiphyses in the head of the first metacarpal and bases of some of the other metacarpals. - It is to be understood that normally the metacarpal bones of the hand have only a single secondary center of ossification; one secondary center is present in the base of the first metacarpal and one each in the heads of all the other metacarpals. - Many times an aberrant epiphysis is found to be present in the distal end (head) of the first metacarpal, and sometimes one or more of the other metacarpals have aberrant epiphyses in their bases. ## Development of Bones - Bones develop through a process called **osteogenesis** (also called **ossification**). - Since the primitive skeleton of the human embryo is composed of either fibrous membranes or hyaline cartilage, bones can form in the embryo in two ways: **intramembranous osteogenesis** and **intracartilaginous or endochondral osteogenesis**. ### Intramembranous Osteogenesis - In this variety of bone formation the immature embryonic connective tissue, called **mesenchyme**, forms layers or "membranes" within which bone-forming cells called **osteoblasts** lay down unmineralized bone matrix called **osteoid**. - The osteoid quickly becomes mineralized by the deposition of calcium phosphate crystals. - The mineralization (calcification) of the osteoid occurs under the influence of an enzyme called **alkaline phosphatase**, which is also secreted by the osteoblasts. - The intramembranous ossification is a direct and quick method of bone formation and the bones developing by this method are called **membrane bones**. - Flat bones of the skull develop by this method of osteogenesis to provide support and protection to the developing brain. - Bones of the face and clavicle also develop by the intramembranous method of osteogenesis. ### Intracartilagineous (Endochondral) Osteogenesis - In this type of bone formation, a miniature cartilaginous model of the bone is first formed. - In this model of hyaline cartilage, bone formation begins in the center of the model as **primary center of ossification**. - Later on, one or more **secondary centers of ossification** appear in the bone ends. - However, it is important to note that the secondary centers of ossification do not appear simultaneously; the secondary center or centers in one end (upper or lower) appear first, while the secondary center of centers in the other end appear a few months or a few years later. - It should be clearly understood that in the endochondral osteogenesis, the hyaline cartilage is not converted into bone but is completely replaced by the bone (osseous) tissue. ## Bone Growth and Remodelling - When a baby is born, bone formation is still incomplete. - In intramembranous bones there is still considerable membrane left around each bone, so that the skull can continue to grow. - Endochondral bones still retain some cartilage. - To be particularly mentioned is a cartilaginous plate that remains between the bony diaphysis and each epiphysis of the long bones. - This cartilage is called **epiphyseal cartilage** or **growth plate**. - Some cartilage also remains at the articular surfaces of the long bone ends. - The epiphyseal cartilage continues to grow, and new bone is actively formed at the diaphyseal end of the growth plate. - In this way, the bone continues to grow in length during the childhood and early adulthood of the person. - The osteoblasts in the inner layer of the periosteum also form bone, so that the bone grows in diameter as well. - As the bone diameter increases, bone tissue is removed from inside the medullary cavity by the activity of the bone-resorbing cell called **osteoclasts**. - As the adulthood arrives, the epiphyseal cartilages of bones stop growing, become narrow and finally disappear, SO that the bone mass of each epiphysis fuses with the bone of the diaphysis; this is called "closure of the epiphysis". ## Rule of Ossification - It has been observed that the long bones of the body ossify according to a general rule of ossification. - According to this rule, that end of a long bone which develops from the first-appearing secondary center of ossification fuses with the diaphysis later than the other epiphysis. - From this observation, a hypothesis, called **growing end hypothesis**, was framed. - According to this hypothesis, one of the two bone ends of a growing long bone grows for a longer period than the other end and, therefore, is called the growing end of the bone. - Generally, the growing end of a long is that end of the bone which ossifies from that secondary center of ossification which appeared first; in simpler words, the rule of ossification states that "the epiphysis that develops from the first-appearing secondary center of ossification is the last to join the diaphysis". - Due to the difference in the growth rates of the two ends of a long bone, the nutrient foramina of these bones are always directed away from the growing ends. - In the upper limb, the upper end of the humerus and lower ends of the ulna are the growing ends, whereas in the lower limb, the upper end of the tibia and lower end of the femur are the growing ends. - As the nutrient foramina of the long bones are directed away from the growing ends, the nutrient foramen of the humerus runs in a downward direction, while the nutrient foramina of the radius and ulna run upwards. - On the other hand, the nutrient foramen of the femur runs in an upward direction, whereas that of the tibia runs downwards. - To facilitate the memorization of the direction of nutrient foramina in the long bones of the limbs an anatomical proverb was devised which says: from the knee I flee, toward the elbow, I go. - It is important to note that the fibula violates the rule of ossification. - In this bone, the secondary center of ossification appears in its distal (lower) end in the first year of life and in its proximal (upper) end in the third year of life. - Violating the general rule of ossification, the distal epiphysis joins the diaphysis earlier (in the 15th year in girls and in the 17th year in boys), whereas the proximal end joins the diaphysis later (in the 17th year in girls and in the 19th year in boys). - Consequently, the nutrient foramen of the fibula is directed upwards and does not "flee from the knee". ## Blood Vessels of Bones ### Blood Supply of Long Bones #### Arterial Supply - Each long bone receives at least one **nutrient artery**, which is usually derived from the major artery of the region. - The nutrient artery (also called diaphyseal artery) enters the bone through the nutrient foramen near the middle of the bone shaft. - In addition to the nutrient artery, a long bone receives blood from three other sets of arteries, which enter the bone through numerous small vascular foramina situated in different parts of the bone. - These arteries are **metaphyseal arteries**, **epiphyseal arteries** and **periosteal arteries**. - The course of different arteries within the bone, their branches and mode of termination are given below. - The **nutrient artery** enters the shaft of the bone through the nutrient foramen, runs obliquely through the bone cortex in the nutrient canal, and then passes into the medullary cavity where it divides into ascending and descending branches. - These branches, referred to as **medullary arteries**, run toward the bone ends. - As they approach the epiphyses, the medullary arteries divide repeatedly and give rise to numerous terminal branches. - Many of these branches are end arteries that drain into venous capillaries which open into a large central venous sinus located in the central part of the medullary cavity. - But some of the branches of medullary arteries anastomose with the terminal branches of the metaphyseal and epiphyseal arteries. - This constitutes an important collateral route for the supply of blood to the bone shaft if a medullary artery is damaged in the event of bone fracture. - Along their course, the medullary arteries give off **central branches** and **cortical branches**. - The central branches deliver blood into a meshwork of medullary sinusoids, which lie in the bone marrow. - These sinusoids drain into the central venous sinus. - The cortical branches of the medullary arteries give rise to capillaries that pass into the bone cortex to run in the haversian canals. - At bone surfaces, the cortical capillaries are connected to the periosteal plexus of capillaries. - The **metaphyseal arteries** are numerous vessels, which are direct branches of major regional arteries coursing near the bone ends. - They pass into the bone through many vascular foramina situated just inner to the epiphyseal line. - Inside the bone the metaphyseal arteries divide into branches which anastomose with the terminal branches of medullary arteries and, in the adult bone, with the branches of the epiphyseal arteries. - The **epiphyseal arteries** are derived from arterial anastomoses, which are formed around the joints by the branches of neighboring major arteries. - At each bone end many epiphyseal arteries enter the bone through many vascular foramina located outer to the epiphyseal line. - Within the spongy bone of the epiphysis, the epiphyseal arteries divide into branches, which anastomose with each other to form many arterial arcades. - From these arcades arise end-arterial loops, most of which return blood to the epiphyseal venous sinusoids. - However, some of these end-arterial loops pierce the thin compact bone present under the articular cartilage to lie in the calcified zone of this cartilage. - These arterial loops may have some role in the nutrition of the deep parts of the articular cartilage. - As mentioned earlier, in the adult bone branches of the epiphyseal arteries also anastomose freely with those of the metaphyseal arteries. - But no such anastomoses exist in the immature (growing) bones because the epiphyseal and metaphyseal arteries cannot link up with each other due to the presence of the intervening epiphyseal cartilage. - The **periosteal arteries** are derived from the arteries of those muscles which are attached to the bone. - Branches arising from the muscular arteries form plexuses over and within the periosteum. - Arterial twigs arising from these plexuses enter the compact bone of the shaft at many points and anastomose with cortical capillaries running in the Haversian canals. - Thus, in addition to providing nutrition of the periosteum, the periosteal arteries aid in the blood supply of the outer part of the bone cortex. #### Venous Drainage of Long Bones - As mentioned above, a large venous sinus is located in the center of the medullary cavity. - This sinus receives blood from the meshwork of medullary sinusoids, which are fed by the central branches of the medullary arteries. - This sinus also receives veins, which return blood from the end-arterial loops formed by the terminal branches of the medullary arteries. - The central venous sinus drains through the nutrient foramen by means of a diaphyseal vein that accompanies the nutrient artery. - In the periosteum a periosteal venous plexus also exists which drains into the venules of the muscles attached to the bone. - The cortical capillaries are connected with the periosteal venous plexus and thus drain their blood into the muscular veins. - This shows that in the shaft of the bone, blood flow through the cortical compact bone occurs centrifugally from the center toward periphery (the cortical branches of the medullary arteries feed cortical capillaries which drain into the periosteal veins). - The spongy bone of the bone ends is drained by many metapyseal and epiphyseal veins, which pass out through numerous vascular foramina to join the neighboring veins. ### Blood Supply of Short Bones - A short bone receives numerous fine blood vessels from the periosteum which enter through the non-articular surfaces of the bone. - These vessels supply the compact and spongy bone, and bone marrow. - The venous drainage occurs by one or two veins, which leave through vascular foramina on the non-articular surfaces of the bone. ### Blood Supply of Flat Bones - The flat bones of the skull receive arterial blood by numerous fine vessels, which originate from the periosteal plexuses. - The veins are large and thin-walled; they run in tortuous canals in diploe. - A rib receives blood from periosteal arteries and a nutrient artery that enters the bone just beyond the tubercle. ### Blood Supply of Irregular Bones - Large irregular bones, like scapulae and hip bones, receive many vessels from the periosteal plexus. - In addition, they are frequently provided with large nutrient arteries, which penetrate through the compact bone into the cancellous bone. - Branches of both systems (periosteal and nutrient) anastomose with each other freely. - Their veins leave from the surface of the bone. - A vertebra receives arterial supply by two arteries, one entering the bone close to the base of each transverse process. - The spongy bone (containing red marrow) drains via two large vessels, called **basivertebral veins**, which converge to a foramen on the posterior surface of the vertebral body. ## Bone Injury and Repair ### Fracture - A break in the continuity of a bone is known as a **fracture**. - A **closed fracture** (also called **simple fracture**) does not produce an open wound in the skin. - An **open fracture** (also called **compound fracture**) is associated with an external wound.