Bone Anatomy PDF

Bursae Bursa is closed sacs or envelopes of serous membrane, Bursae are normally collapsed. their walls are apposed with only a thin film of lubricating fluid between them that is secreted by the enclosing membranes. Usually occurring in locations subject...

Bursae Bursa is closed sacs or envelopes of serous membrane, Bursae are normally collapsed. their walls are apposed with only a thin film of lubricating fluid between them that is secreted by the enclosing membranes. Usually occurring in locations subject to friction, bursae enable one structure to move more freely over another. Subcutaneous bursae occur in the subcutaneous tissue between the skin and bony prominences, such as at the elbow or knee. subfascial bursae lie beneath deep fascia. subtendinous bursae facilitate the movement of tendons over bone. Synovial tendon sheaths are a specialized type of elongated bursae that wrap around tendons, usually enclosing them as they traverse osseofibrous tunnels that anchor the tendons in place Bursae occasionally communicate with the synovial cavities of joints. Collapsed bursal sacs surround many important organs (e.g., the heart, lungs, and abdominal viscera) and structures (e.g., portions of tendons) configuration is much like wrapping a large but empty balloon around a structure, such as a fist The object is surrounded by the two layers of the empty balloon but is not inside the balloon; the balloon itself remains empty. the heart is surrounded by— but is not inside—the pericardial sac. Each lung is surrounded by—but is not inside—a pleural sac, and the abdominal viscera are surrounded by—but are not inside—the peritoneal sac. (the one adjacent to the fist, viscus, or viscera) is called the visceral layer; the outer layer of the balloon (or the one in contact with the body wall) is called the parietal layer. 1 Synovial tendon sheaths and bursal sacs. A. Synovial tendon sheaths. Longitudinal bursae surround tendons as they pass deep to retinacula or through fibrous digital sheaths. B. Bursal sacs. Several structures, such as the heart, lungs, abdominal viscera, and tendons, are enclosed much like this collapsed balloon encloses the fist. A thin film of lubricating fluid between the parietal and visceral layers confers mobility to the structure surrounded by the bursa within a confined compartment. The transitional folds of synovial membrane between the continuous parietal and visceral layers surrounding the connecting stalks (the wrist in this example) and/or neurovascular structures serving the surrounded mass are called mesenteries. In the case of a synovial tendon sheath, the mesentery is called a mesotendon. 2 The skeletal system may be divided into two functional parts: The axial skeleton consists of the bones of the head (cranium or skull), neck (hyoid bone and cervical vertebrae), and trunk (ribs, sternum, vertebrae, and sacrum). The appendicular skeleton consists of the bones of the limbs, including those forming the pectoral (shoulder) and pelvic girdles. Cartilage and Bones The skeleton is composed of cartilages and bones. Cartilage is a resilient, semirigid form of connective tissue that forms parts of the skeleton where more flexibility is required A fibrous membrane called the perichondrium covers the cartilage except on the exposed surfaces in joints. Of the three types of cartilage it plays an important part in the growth in length of long bones ( ). It has a great resistance to wear and covers the articular surfaces of nearly all synovial joints. Hyaline cartilage Is Incapable of repair when fractured; the defect is filled with fibrous tissue. 3  has many collagen fibers embedded In a small amount of matrix and Is found In the discs within joints (e.g., the temporomandibular joint, sternoclavicular joint, and knee joint) and on the articular surfaces of the clavicle and mandible. Fibrocartilage, if damaged, repairs itself slowly in a manner similar to fibrous tissue elsewhere. Joint discs have a poor blood supply and therefore do not repair themselves when damaged. possesses large numbers of elastic fibers embedded In matrix, making It flexible. It Is found in the auricle of the ear, the external auditory meatus, the auditory tube, and the epiglottis. Elastic cartilage, if damaged, repairs itself with fibrous tissue. Blood vessels do not enter cartilage (i.e., it is avascular); consequently, its cells obtain oxygen and nutrients by diffusion. The proportion of bone and cartilage in the skeleton changes as the body grows Bone, a living tissue, is a highly specialized, hard form of connective tissue that makes up most of the skeleton. Bones of the adult skeleton provide: Support for the body and its vital cavities; it is the chief supporting tissue of the body. Protection for vital structures (e.g., the heart). The mechanical basis for movement (leverage). Storage for salts (e.g., calcium). A continuous supply of new blood cells (produced by the marrow in the medullary cavity of many bones) A fibrous connective tissue covering surrounds each skeletal element like a sleeve, except where articular cartilage occurs; that surrounding bones is periosteum. The two types of bone are compact bone and spongy (trabecular) bone. They are distinguished by the relative amount of solid matter and by the number and size of the spaces they contain All bones have a superficial thin layer of compact bone around a central mass of spongy bone, except where the latter is replaced by a medullary (marrow) cavity. Within the medullary cavity of adult bones, and between the spicules (trabeculae) of spongy bone, yellow (fatty) or red (blood cell and platelet forming) bone marrow—or a combination of both— is found. Transverse sections of femur. The shaft of a living bone is a tube of compact bone that surrounds a medullary cavity. 4 Classification of Bones Bones are classified according to their shape. Long bones are tubular (e.g., the humerus in the arm). Short bones are cuboidal and are found only in the tarsus (ankle) and carpus (wrist). Flat bones usually serve protective functions (e.g., the flat bones of the cranium protect the brain) and The scapulae. Irregular bones have various shapes other than long, short, or flat (e.g., bones of the face vertebrea and pelvic bones). Sesamoid bones (e.g., the patella or knee cap) develop in certain tendons and are found where tendons cross the ends of long bones in the limbs; they protect the tendons from excessive wear. Bone Markings and Formations Bone markings appear wherever tendons, ligaments, and fascias are attached or where arteries lie adjacent to or enter bones. Condyle: rounded, knuckle-like articular area, often occurring in pairs (e.g., the lateral and medial femoral condyles). Crest: ridge of bone (e.g., the iliac crest). Epicondyle: eminence superior or adjacent to a condyle (e.g., lateral epicondyle of the humerus). Facet: smooth flat area, usually covered with cartilage, where a bone articulates with another bone (e.g., superior costal facet on the body of a vertebra for articulation with a rib). Foramen: passage through a bone (e.g., obturator foramen). Fossa: hollow or depressed area (e.g., infraspinous fossa of the scapula). Groove: elongated depression or furrow (e.g., radial groove of the humerus). Head : large, round articular end (e.g., head of the humerus). Line: linear elevation, sometimes called a ridge (e.g., soleal line of the tibia). Malleolus: rounded process (e.g., lateral malleolus of the fibula). Neck: relatively narrow portion adjacent to the head. Process: ( spinous process, or transverse process of a vertebra ). Shaft: the diaphysis, or body, of a long bone. Spine: thorn-like process (e.g., the spine of the scapula). Trochanter: large blunt elevation (e.g., greater trochanter of the femur). Trochlea: spool-like articular process or process that acts as a pulley (e.g., trochlea of the humerus). Tubercle: small raised eminence (e.g., greater tubercle of the humerus). Tuberosity: large rounded elevation (e.g., ischial tuberosity). 5  Bone markings and formations. Markings appear on bones wherever tendons, ligaments, and fascia attach. Other formations relate to joints, the passage of tendons, and the provision of increased leverage. 6 Bone Development Most bones take many years to grow and mature. The humerus (arm bone), for example, begins to ossify at the end of the embryonic period (8 weeks); however, ossification is not complete until age 20. All bones derive from mesenchyme (embryonic connective tissue) by two different processes: intramembranous ossification (directly from mesenchyme) and endochondral ossification (from cartilage derived from mesenchyme). The two processes of bone development proceed as follows: In intramembranous ossification (membranous bone formation), mesenchymal models of bones form during the embryonic period, and direct ossification of the mesenchyme begins in the fetal period. In endochondral ossification (cartilaginous bone formation), cartilage models of the bones form from mesenchyme during the fetal period, and bone subsequently replaces most of the cartilage. A brief description of endochondral ossification helps explain how long bones grow. The mesenchymal cells condense and differentiate into chondroblasts, dividing cells in growing cartilage tissue, thereby forming a cartilaginous bone model. The shaft of a bone ossified from the primary ossification center is the diaphysis, which grows as the bone develops. Development and growth of a long bone. A. Ossification centers. The formation of primary and secondary ossification centers is shown. B. Growth of long bones. Growth in length occurs on both sides of the cartilaginous epiphysial plates (arrowheads). The bone formed from the primary center in the diaphysis does not fuse with that formed from the secondary centers in the epiphyses until the bone reaches its adult size. When growth ceases, the depleted epiphysial plate is replaced by a synostosis (bone-to-bone fusion), observed as an epiphysial line in radiographs and sectioned bone. 7 Most secondary ossification centers appear in other parts of the developing bone after birth; the parts of a bone ossified from these centers are epiphyses. The flared part of the diaphysis nearest the epiphysis is the metaphysis. For growth to continue, the bone formed from the primary center in the diaphysis does not fuse with that formed from the secondary centers in the epiphyses until the bone reaches its adult size. Thus, during growth of a long bone, cartilaginous epiphysial plates intervene between the diaphysis and epiphyses These growth plates are eventually replaced by bone at each of its two sides, diaphysial and epiphysial. When this occurs, bone growth ceases and the diaphysis fuses with the epiphyses. The seam formed during this fusion process (synostosis) is particularly dense and is recognizable in sectioned bone or radiographs as an epiphysial line 8 Vasculature and Innervation of Bones Bones are richly supplied with blood vessels. Most apparent are the nutrient arteries (one or more per bone) that arise as independent branches of adjacent arteries outside the periosteum Veins accompany arteries through the nutrient foramina. Nerves accompany blood vessels supplying bones. The periosteum is richly supplied with sensory nerves—periosteal nerves—that carry pain fibers. The periosteum is especially sensitive to tearing or tension, which explains the acute pain from bone fractures. Bone itself is relatively sparsely supplied with sensory endings. 9