Medical brain notes 27

  The Skeletal System I magine for a moment that people did not have skeletons. What comes to mind? Probably that each of us would be a little heap on the floor, much like a jel-lyfish out of water. Such an image is accurate and reflects the most obvious function of the skeleton: to support the body. Although it is a framework for the body, the skeleton is not at all like the wooden beams that support a house. Bones are living organs that actively contribute to the maintenance of the internal environment of the body.   The  skeletal system  consists of bones and other structures that make up the joints of the skeleton. The types of tissue present are bone tissue, cartilage, and fibrous connective tissue, which forms the ligaments that connect bone to bone.

  FUNCTIONS OF THE SKELETON   1.             Provides a framework that supports the body; the muscles that are attached to bones move the skeleton.   2.             Protects some internal organs from mechanical injury; the rib cage protects the heart and lungs, for example.   3.             Contains and protects the red bone marrow, the primary hemopoietic (blood-forming) tissue.   4.             Provides a storage site for excess calcium. Calcium may be removed from bone to maintain a normal blood calcium level, which is essential for blood clotting and proper functioning of muscles and nerves.

  TYPES OF BONE TISSUE   Bone  was described as a tissue. Recall   that bone cells are called  osteocytes , and the  matrix  of bone is made of  calcium salts  and collagen. The calcium salts are calcium carbonate (CaCO 3 ) and cal-cium phosphate (Ca 3 (PO 4 ) 2 ), which give bone the strength required to perform its supportive and pro-tective functions. Bone matrix is non-living, but it changes constantly, with calcium that is taken from bone into the blood replaced by calcium from the diet. In normal circumstances, the amount of calcium that is removed is replaced by an equal amount of calcium deposited. This is the function of osteocytes, to regu- late the amount of calcium that is deposited in, or removed from, the bone matrix.   In bone as an organ, two types of bone tissue are present (Fig. 6–1).  Compact bone  looks solid but is very precisely structured. Compact bone is made of  osteons  or  haversian sys...

  CLASSIFICATION OF BONES   1.   Long bones —the bones of the arms, legs, hands,   and feet (but not the wrists and ankles). The shaft of a long bone is the  diaphysis , and the ends are called  epiphyses  (see Fig. 6–1). The diaphysis is made of compact bone and is hollow, forming a canal within the shaft. This  marrow canal  (or medullary cavity) contains  yellow bone marrow , which is mostly adipose tissue. The epiphyses are made of spongy bone covered with a thin layer of compact bone. Although red bone marrow is pres-ent in the epiphyses of children’s bones, it is largely replaced by yellow bone marrow in adult bones.   2.   Short bones —the bones of the wrists and ankles.   3.   Flat bones —the ribs, shoulder blades, hip bones,   and cranial bones. 4.   Irregular bones —the vertebrae and facial bones. Figure 6–1.  Bone tissue. ( A ) Femur with distal end cut in longitudinal section.   ( B ) C...

  EMBRYONIC GROWTH OF BONE   During embryonic development, the skeleton is first made of cartilage and fibrous connective tissue, which are gradually replaced by bone. Bone matrix is pro-duced by cells called  osteoblasts  (a  blast  cell is a “grow-ing” or “producing” cell, and  osteo  means “bone”). In the embryonic model of the skeleton, osteoblasts dif-ferentiate from the fibroblasts that are present. The production of bone matrix, called  ossification , begins in a  center of ossification  in each bone.   The cranial and facial bones are first made of fibrous connective tissue. In the third month of fetal development, fibroblasts (spindle-shaped connective tissue cells) become more specialized and differentiate into osteoblasts, which produce bone matrix. From each center of ossification, bone growth radiates out-ward as calcium salts are deposited in the collagen of the model of the bone. This process is not complete at b...

  FACTORS THAT AFFECT BONE GROWTH AND MAINTENANCE   1.   Heredity—each person has a genetic potential for height, that is, a maximum height, with genes inherited from both parents. Many genes are involved, and their interactions are not well under-stood. Some of these genes are probably those for the enzymes involved in cartilage and bone pro-duction, for this is how bones grow.   2.   Nutrition—nutrients are the raw materials of which bones are made. Calcium, phosphorus, and protein become part of the bone matrix itself. Vitamin D is needed for the efficient absorption of calcium and phosphorus by the small intestine. Vitamins A and C do not become part of bone but are necessary for the process of bone matrix forma-tion (ossification). Without these and other nutri-ents, bones cannot grow properly. Children who are malnourished grow very slowly and may not reach their genetic potential for height.   3.   Hormones—endocrine glands produce hormones that...

  THE SKELETON   The human skeleton has two divisions: the  axial skele-ton , which forms the axis of the body, and the  appen-dicular skeleton , which supports the appendages or   limbs. The axial skeleton consists of the skull, vertebral column, and rib cage. The bones of the arms and legs and the shoulder and pelvic girdles make up the appendicular skeleton. Many bones are connected to other bones across joints by ligaments, which are strong cords or sheets of fibrous connective tissue. The importance of ligaments becomes readily apparent when a joint is sprained. A sprain is the stretching or even tearing of the ligaments of a joint, and though the bones are not broken, the joint is weak and unsteady. We do not often think of our ligaments, but they are necessary to keep our bones in the proper positions to keep us upright or to bear weight.   There are 206 bones in total, and the complete skeleton is shown in Fig. 6–4.

  SKULL   The  skull  consists of 8 cranial bones and 14 facial bones. Also in the head are three small bones in each middle ear cavity and the hyoid bone that supports the base of the tongue. The  cranial bones  form the brain-case (lined with the meninges) that encloses and pro-tects the brain, eyes, and ears. The names of some of these bones will be familiar to you; they are the same as the terminology used  to describe areas of the head. These are the frontal bone, parietal bones (two), temporal bones (two), and occipital bone. The sphenoid bone and ethmoid bone are part of the floor of the braincase and the orbits (sockets) for the eyes. The  frontal bone  forms the forehead and the anterior part of the top of the skull.  Parietal  means “wall,” and the two large  parietal bones  form the posterior top and much of the side walls of the skull. Each  temporal bone  on the side of the skull contains an   e...

  VERTEBRAL COLUMN   The  vertebral column  (spinal column or backbone) is made of individual bones called  vertebrae . The names of vertebrae indicate their location along the length of the spinal column. There are 7 cervical vertebrae, 12 thoracic, 5 lumbar, 5 sacral fused into 1 sacrum, and 4 to 5 small coccygeal vertebrae fused into 1 coccyx (Fig. 6–10).   The seven  cervical vertebrae  are those within the neck. The first vertebra is called the  atlas , which artic-ulates with the occipital bone to support the skull and forms a  pivot joint  with the odontoid process of the  axis , the second cervical vertebra. This pivot joint   allows us to turn our heads from side to side. The remaining five cervical vertebrae do not have individ-ual names.   The  thoracic vertebrae  articulate (form joints) with the ribs on the posterior side of the trunk. The  lumbar vertebrae , the largest and strongest bones...

  RIB CAGE   The  rib cage  consists of the 12 pairs of ribs and the sternum, or breastbone. The three parts of the ster-num are the upper manubrium , the central  body , and the lower  xiphoid process  (Fig. 6–11).   All of the  ribs  articulate posteriorly with the tho-racic vertebrae. The first seven pairs of ribs are called  true ribs ; they articulate directly with the manubrium   and body of the sternum by means of costal cartilages. The next three pairs are called  false ribs ; their carti- lages join the 7th rib cartilage. The last two pairs are called  floating ribs  because they do not articulate with the sternum at all (see Fig. 6–10).   An obvious function of the rib cage is that it en-closes and protects the heart and lungs. Keep in mind, though, that the rib cage also protects organs in the upper abdominal cavity, such as the liver and spleen. The other important function of the rib cage depend...

  THE SHOULDER AND ARM   The shoulder girdles attach the arms to the axial skele-ton. Each consists of a scapula (shoulder blade) and clavicle (collarbone). The scapula  is a large, flat bone with several projections (the spine of the scapula, the coracoid process) that anchor some of the muscles that move the upper arm and the forearm. A shallow depression called the glenoid fossa forms a  ball-and-socket joint  with the humerus, the bone of the upper   arm (Fig. 6–12).   Each  clavicle  articulates laterally with a scapula and medially with the manubrium of the sternum. In this position the clavicles act as braces for the scapulae and prevent the shoulders from coming too far for-ward. Although the shoulder joint is capable of a wide range of movement, the shoulder itself must be rela-tively stable if these movements are to be effective.   The  humerus  is the long bone of the upper arm. In Fig. 6–12, notice the deltoid tuber...

  THE HIP AND LEG   The pelvic girdle (or pelvic bone) consists of the two  hip bones  (coxae or innominate bones), which articu-late with the axial skeleton at the sacrum. Each hip bone has three major parts: the ilium, ischium, and pubis, and these are shown in Fig. 6–13, which depicts both a male and a female pelvis. The  ilium  is the flared, upper portion that forms the sacroiliac joint. The  ischium  is the lower, posterior part that we sit on. The  pubis  is the lower, most anterior part. The two pubic bones articulate with one another at the  pubic symphysis , with a disc of fibrous cartilage   between them. Notice the pubic angle of both the male and female pelvises in Fig. 6–13. The wider female angle is an adaptation for childbirth, in that it helps make the pelvic outlet larger.   Figure 6–13.  Hip bones and sacrum. ( A ) Male pelvis, anterior view. ( B ) Male pelvis, lat-eral view of right side. ( C ) Fem...