The Skeleton : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 16 of 28)

The pelvic girdle consists of two large bones of irregular shape, called the innominate bones. They connect behind with the sacrum and in front they connect, through a small pad of cartilage, with each other. On the inside of the girdle is a smooth, basin-shaped support for the contents of the abdomen, but on the outside the bones are rough and irregular and provide many places for the attachment of muscles and ligaments. Each innominate bone has a deep, round socket into which the end of the femur (the long bone of the leg) accurately fits.

5. The Arm and Hand Groups. - A long bone, the humerus, connects the arm with the shoulder and gives form to the upper arm. In the forearm are two bones, the radius and the ulna, which connect at one end with the humerus and at the other with the bones of the wrist.

A group of eight small, round bones is found in the wrist, known as the carpal bones. These are arranged in two rows and are movable upon one another. Five straight bones, the metacarpals, connect with the wrist bones and form the framework for the palm of the hand. Attached to the metacarpals are the bones of the fingers and thumb. These form an interesting group of fourteen bones, called the phalanges of the fingers.

The bones of the hand provide a mechanical device, or machine, for grasping, and the arm serves as a device for moving this grasping machine from place to place. The work of the arm, in this respect, is not unlike that of a revolving crane upon the end of which is a grab-hook. The hand without the arm to move it about would be of little use.

6. The Leg and Foot Groups. - These correspond in form and arrangement to the bones of the arm and hand. Since, however, the leg and foot are used for purposes different from those of the arm and hand, certain differences in structure are to be found. The patella, or kneepan, has no corresponding bone in the arm; and ankle, which corresponds to the wrist, contains seven instead of eight bones. The bones of the foot and toes are the same in number as those of the hand and fingers, but they differ greatly in size and form and have less freedom of motion. The femur, which gives form to the thigh, is the longest bone of the body. The tibia, or shin bone, and the fibula, the slender bone by its side, give form to the lower part of the leg.

The legs are mechanical devices (walking machines) for moving the body from place to place. The feet serve both as supports for the body and as levers for pushing the body forward. By their attachment to the legs they may be placed in all necessary positions for supporting and moving the body.

Adaptation to Special Needs. - When any single bone is studied in its relation to the other members of the group to which it belongs or with particulary reference to its purpose in the body, its adaptation to some special place or use is at once apparent. Each bone serves some special purpose, and to this purpose it is adapted by its form and structure. Long bones, like the humerus and femur, are suited to giving strength, form, and stiffness to certain parts, while irregular bones, like the vertebra and the pelvic bones, are fitted for supporting and protecting organs. Others, like the wrist and ear bones, make possible a peculiar kind of motion, and still others, like the ribs, are adapted to more than one purpose. The vast differences in shape, size, structure, and surface among the various bones are but the conditions that adapt them to particulary forms of service in the body.

Articulations

Any place in the body where two or more bones meet is called an articulation, or joint. At the place of meeting the bones are firmly attached to each other, thereby securing the necessary coherence of the skeleton. The large number of bones, and consequently of articulations, are necessary for the different movements of the body and also on account of the manner in which the skeleton develops, or grows. Articulations are classed with reference to their freedom of motion, as movable, slightly movable, and immovable articulations.

Most of the immovable articulations are found in the skull. Here irregular, tooth-like projections from the different bones enable them to interlock with one another, while they are held firmly together by a thin layer of connective tissue. The wavy lines formed by articulations of this kind are called sutures.

The best examples of joints that are slightly, but not freely, movable are found in the front of the spinal column. The cartilaginous pads between the vertebra permit, by their elasticity, of a slight bending of the column in different directions. These movements are caused, not by one bone gliding over another, but by compressions and extensions of the cartilage. Between the vertebra in the back of the spinal column, however, there is a slight movement of the bone surfaces upon one another.

Structure of the Movable Joints. - By far the most numerous and important of the joints are those that are freely movable. Such joints are strongly constructed and endure great strain without dislocation, and yet their parts move over each other easily and without friction. The ends of the bones are usually enlarged and have specially formed projections or depressions which fit into corresponding depressions or elevations on the bones with which they articulate. In addition to this the articulary surfaces are quite smooth and dense, having no Haversian canals, and they are covered with a layer of cartilage. Strong ligaments pass from one bone to the other to hold each in its place. Some of these consist simply of bands, connecting the joint on its different sides, while others form continuous sheaths around the joint.

The interior of the joint, except where the bone surfaces rub upon each other, is covered with a serous lining, called the synovial membrane. This secretes a thick, viscid liquid, the synovial fluid, which prevents friction. The synovial membrane does not cover the ends of the bones, but passes around the joint and connects with the bones at their edges so as to form a closed sac in which the fluid is retained.

D.C. Heath and Co. - Publishers
Original copyright 1909