VII. The Skeletal System

This is a system which is made up of organs termed bones. A bone is a living, dynamic organ which contains not only bony tissue but has a circulation and nervous connections as well. The skeletal system is very closely associated with the skeletal muscle system, most of which attach either directly or indirectly to the skeleton. Sometimes the two systems are considered together as the musculoskeletal system.

A. Functions

l. Provides the framework and support for the rest of the body.

2. It protects the soft organs.

3. It is the site of hemopoiesis, the generation of blood cells.

4. Calcium metabolism - The skeleton is the major repository for calcium in the body.

5. Movement (leverage) - Movable joints permit the muscular system to cause the skeleton, and therefore the body, to move.

B. Bone types - Based upon their shape bones can be divided into five categories.

l. Long bones - Longer than wide, these are the bones of the legs, arms, and fingers.

2. Short bones - These are cube shaped bones found in the ankle and wrists.

3. Flat bones - These are sheet-like and include the bones of the cranium and the ribs.

4. Irregular bones - These have no distinct shape. These include the bones of the face, the vertebrae, and the girdles.

5. Sesamoid bones - The are bones that form inside of tendons. They vary in number and placement from person to person, but the patellas or knee caps are found in everyone.

6. Wormian (sutural) bones - These are bones that form between suture lines. They vary in number and position.

C. Bone structure - Bones are complex organs with both a gross and microscopic structure.

l. Gross structure of a typical long bone

a. External anatomy - A typical long bone has two heads or ends known as epiphyses. These are connected by a shaft termed the diaphysis. The bone is covered by a membrane known as the periosteum.

b. Internal anatomy - There are two major types of bony tissue.

(l) Compact bone - This is very dense bone which coats the entire structure.

(2) Cancellous (spongy) bone - This highly porous bone makes up most of the epiphyses.

(3) Marrow cavity (medullary cavity) - This is a cavity found in the center of the diaphysis. It is lined by a membrane termed the endosteum. It is filled with one of two kinds of marrow.

(a) Red marrow - This fills the cavity during embryology and early life. It generates blood cells.

(b) Yellow marrow - This consists of fat and fills the marrow cavities in the bones of adults.

(4) Epiphyseal plates - These are cartilaginous growth plates that are found in the epiphyses. They permit the long bones to continue to grow throughout childhood and adolescence. At the end of the growth period they become ossified and growth in the length of the bone ceases.

c. Bone markings - These are bumps, ridges, projections, holes, etc., which are found on bones. They normally have some function such as passage for a blood vessel, attachment for a tendon, or some other item. A set of terms has been developed to describe these markings. They are listed on page 180 of your textbook. You should commit them to memory.

2. Microscopic structure of bone

a. Compact bone - This form is composed of tightly packed cylindrical structures termed Haversian systems or Osteons. Each osteon is structured as follows.

(1) There is a central canal or Haversian canal in the center of each osteon. A blood vessel surrounded by tissue fluid is found in this canal.

(2) Radiating off of the canal are fine channels known as canaliculi. These communicate with the lacunae where the bone cells (osteocytes) lie. This permits tissue fluid which has exchanged materials with the blood vessel to bath the cells.

(3) The osteocyte containing lacunae are found in concentric circles around the central canal. The bone cells lay down layers of bone to either side of them forming bony lamellae which separate the concentric circles of lacunae.

b. Cancellous bone - This consists of spicules of bone with lacunae and canaliculi, but no organized osteon. The spicules interconnect leaving spaces which are usually filled with red marrow.

D. Ossification - This is the process of bone formation. There are two major types of bone formation, intramembranous and endochondral.

1. Intramembranous (membrane bone formation) - This occurs only in the skull. The process is as follows.

a. Two parallel periosteal membranes bud off bone cells (osteoblasts) between them.

b. Osteoblasts are active bone forming cells and begin to secrete bony tissue around themselves. Eventually they are completely surrounded by bone and lie in lacunae. At this point they stop actively secreting bone and become maintenance cells known as osteocytes.

c. Ossification begins in two or three centers and moves from there.

d. At the time of birth, not all of the ossification centers have met. These leaves purely membranous areas where the process is not complete. These soft spots are called fontanels

2. Endochondral ossification (cartilage replacement) - This is the type of bone formation that occurs in the majority of the skeleton. The process is as follows.

a. A cartilage model of the bone, complete with perichondrium, first forms.

b. At a certain point, the chondrocytes begin to secrete an enzyme which results in mineral deposition and hardening of the cartilage. The hardening of the cartilage blocks the exchange of the chondrocytes with the perichondrium and consequently the chondrocytes begin to die. The death of the chondrocytes leads to degeneration of the cartilage.

c. Concurrent with cartilage death, the perichondrium transforms into a periosteum. This begins to bud off osteoblasts.

d. A bony collar is formed around the diaphysis by the periosteum.

e. A clump of osteoblasts breaks through the collar into the degenerating cartilage and forms a bud which now becomes the primary center of ossification.

f. Additional osteogenic buds invade the epiphyses forming secondary centers of ossification.

g. The last cartilage to be replaced are the epiphyseal plates which continue to grow until the end of adolescence.

h. Bones are rebuilt and reshaped throughout life. A third kind of bone cells, the osteoclast, will break down bone while osteoblasts rebuild it. A good example of this process is the enlargement of the cranial cavity in the skull during childhood.

E. Basic skeletal plan - The skeleton of all vertebrates, including humans, is much like a suspension bridge. There is a horizontal component (axial division - skull and vertebral column) which is supported by two perpendicular components (appendicular division -arms and legs). This pattern is more obvious in four legged animals, but it is also true for humans. The major difference is that humans balance on one set of supports instead of using both sets.

F. Major divisions - The skeleton has two major divisions, the axial division and the appendicular division.

l. Axial division - This forms the long axis of the body and consists of the skull, vertebral column, rib cage, and sternum. There are a total of 80 bones in the typical axial division.

a. Skull - This is composed of 29 bones. It forms a protective box for the brain and special sense organs. It has two major regions.

(l) Cranium - This is the bony box that surrounds the brain. The bones that make it up are joined by immovable joints called sutures.

(2) Face - Irregular shaped bones which are joined by sutures make up this region. It also contains the only movable joint in the skull (tempro-mandibular). The face houses the teeth, most of the nasal cavity, hard palate, and most of the eye orbits.

b. Vertebral column - Typically this consists of 26 bones. The skull is balanced on top of the column with the ribs and viscera suspended in front. The column forms a protective enclosure for the spinal cord.

(1) Regions - There are five regions of the vertebral column.

(a) Cervical (neck) - 7 bones

(b) Thoracic (thorax) - 12 bones

(c) Lumbar (small of back) - 5 bones

(d) Sacrum - l bone

(e) coccyx (tail bone) - l bone

(2) Common features - Although there are differences between vertebrae found in the different regions, all share certain common features.

(a) All except the first have a rounded mass known as the body or centrum.

(b) All have a dorsal projection known as a spine.

(c) All have lateral projections known as the transverse processes.

(d) All except the sacrum and the coccyx have a central opening known as the vertebral foramen.

(e) All adjacent centra are separated by a disk of fibrocartilage.

(3) Unique features - Vertebrae from each region have certain features that are unique to the region to which they belong. These features are listed by region below.

(a) Cervical

/1/ There is a foramen in the transverse processes.

/2/ The spinous process is bifurcated (forked) except for the seventh.

/3/ The first cervical vertebra is known as the atlas and supports the skull. The second vertebra is the axis. It contains a special odontoid process around which the atlas rotates. This permits you to move your head from side to side.

(b) Thoracic

/1/ Each vertebra possess facets on both sides of the centrum and on the ends of both transverse processes. These facets articulate with the ribs.

/2/ There is a sharp, inferior projecting spinous process.

(c) Lumbar

/1/ The superior articulating facet is directed inward while the inferior articulating facet is directed outward. These two sets of facets are directed superiorly and inferiorly respectively in vertebrae from the other regions.

/2/ Each vertebra is heavy and massive with a short, blunt, spinous process.

(d) Sacrum

/1/ This is a single bone which is formed embryologically by the fusion of five separate vertebrae.

/2/ This bone is curved in men but straight in women. This is why women tend to "stick out" in the buttock region while men tend to be flat.

(e) Coccyx - This small tail bone is formed by the fusion of four or five vertebrae. Because the straight sacrum in females causes this bone to project outward, it frequently broken in falling accidents.

(4) Curves - There are three normal curves in the vertebral column. These curves adjust the center of gravity of the body so that standing erect is less tiring than it would be if the column was straight.

(a) Cervical - A concave curve in the neck region.

(b) Thoracic - A convex curve in the thoracic region.

(c) Lumbar - An concave curvature in the lumbar region.

(5) Abnormalities of the vertebral column

(a) Kyphosis (hunchback) - An exaggerated convexity of the thoracic region.

(b) Lordosis (swayback) - An exaggerated concavity of the lumbar region.

(c) Scoliosis - A lateral curvature in any place in the column.

c. Ribs - There are 12 pairs of ribs, each connected to a thoracic vertebra. The ribs are divided into three groups.

(1) True ribs - This is the first seven pairs of ribs. Each has its own costal cartilage which connects directly to the sternum.

(2) False ribs - This is the next three pairs of ribs. Each connects to the sternum indirectly via the costal cartilage of the seventh pair of ribs.

(3) Floating ribs - These are the last two pairs. They have no connection to the sternum at all.

d. Sternum - This is the breastbone, a dagger shaped bone that forms with the ribs and vertebral column the cage which is known as the thorax.

2. Appendicular division - This consists of the appendages and the girdles, a total of 126 bones.

a. Girdles - These function to connect the appendages to the axial division. The upper girdle is the pectoral and the lower girdle is the pelvic.

(l) Pectoral (shoulder) girdle

(a) There is a left and right girdle and each consists of two bones, the scapula (shoulder blade) and the clavicle (collar bone).

(b) The clavicle articulates with the sternum and the acromion process of the scapula.

    1. The clavicle and scapula for a "V" with the humerus (upper arm bone) suspended from the

apex (glenoid cavity of the scapula).

(2) Pelvic (hip) girdle

(a) This is composed of two bones, the os coxae or innominate bones.

(b) Each os coxis joins with the sacrum posteriorly and with each other at the pubic symphysis anteriorly.

(c) The result is a solid ring of bone which is very solidly attached to the axial skeleton.

b. Appendages - There is a fundamental similarity between the arms and legs. The upper part is composed of single large bone while the lower part is made up of two smaller bones. The hands and feet are composed of many small bones, all similarly arranged.

(l) Upper appendage

(a) The upper arm bone is the humerus. It articulates distally with the forearm bones, the radius and ulna.

(b) The radius is located on the thumb side of the arm. Both bones articulates distally with the carpals, eight cube shaped bones that form the wrist.

(c) The carpals articulate with five metacarpals which form the framework of the hand. Metacarpals articulate with the finger bones, the phalanges. There are three phalanges per finger except for the thumb which has two.


(2) Lower appendage

(a) The upper leg bone is the femur, the largest bone in the body. Proximally its ball shaped head fits into the cup-like acetabulum of the os coxae.

(b) The distal end of the femur articulates with the tibia only, the larger of the foreleg bone. The fibula, the smaller foreleg bone, articulates only with the tibia.

(c) The tibia articulates with the ankle which is composed of seven tarsal bones. These join with five metatarsals which form the framework of the foot. Toes are composed of fourteen phalanges as in the hands, three per toe except for the big toe which has two.

(d) Arches - There are two major arches in the foot, the longitudinal and the transverse. These arches are formed by the tarsals and metatarsals. The arches serve to distribute weight over the entire foot instead of just the heel.

G. Fractures - These are breaks in the bone. There are several different kinds. Three of the more common types are as follows.

l. Simple - The bone breaks but stays within the body.

2. Compound - The bone breaks and protrudes through the skin.

3. Greenstick - This is a characteristic fracture of children. Due to the large amount of fibers and lesser quantities of minerals, the bone may break but the ends do not come apart. This is similar to how a green branch breaks.

4. Repair of bone

a. The periosteum in the vicinity of the break forms a mass of tissue at the ends of the break which grow together to form a collar around the break. This collar is termed a callus.

b. In about two weeks spicules of cancellous bone are found adjacent to the break. The callus has now become fibro-cartilage in nature.

c. The fibro-cartilage callus is gradually converted into bone.

d. Eventually the entire area is reshaped and remodeled with the callus being reabsorbed. The healing is now complete.