The Skeleton : Part 2

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 15 of 28)

Plan and Purpose of the Skeleton. - The framework of the body is such as to adapt it to a movable structure. Obviously the different parts of the body cannot be secured to a foundation, as are those of a stationary building, but must be arranged after a plan that is conducive to motion. A moving structure, as a wagon or a bicycle, has within it some strong central part to which the remainder is joined. The same is true of the skeleton. That part to which the others are attached is a long, bony axis, known as the spinal column. Certain parts, as the ribs and the skull, are attached directly to the spinal column, while others are attached indirectly to it. The arrangement of all the parts is such that the spinal column is made the central, cohering portion of the skeleton and also of the whole body.

Besides the general arrangement of the parts of the skeleton, there is such a grouping of the bones in each of its main divisions as will enable them to serve definite purposes. In most places they form mechanical devices for supplying special movements, and in certain places they provide for the support or protection of important organs. In most cases there is a definite combination of different bones, forming what is called the bone group.

Bone Groups. - On account of the close relation between the bones of the same group, they cannot profitably be studied as individual bones, but each must be considered as a part of the group to which it belongs. By first making out the relation of a given bone to its group, its value to the whole body can be determined. The most important of the groups of bones are as follows:

1. The Spinal Column. - This group consists of twenty-four similarly shaped bones, placed one above the other, called the vertebra, and two bones found below the vertebra, known as the sacrum and the coccyx. These twenty-six bones supply the central axis of the body, support the head and upper extremities, and enclose and protect the spinal cord.

The upper seven vertebra form the neck and are called the cervical vertebra. They are smaller and have greater freedom of motion than the others. The first and second cervical vertebra, known as the atlas and the axis, are specially modified to form a support for the head and provide for its movements. The head rests upon the atlas, forming with it a hinge joint (used in nodding to indicate "yes"); and the atlas turns upon an upward projection of the axis forming a pivot joint (used in shaking the head to indicate "no").

The next twelve vertebra, in order below the cervical, are known as the thoracic vertebra. They form the back part of the framework of the thorax and have little freedom of motion. The five vertebra below the thoracic are known as the lumbar vertebra. These bones are large and strong and admit of considerable motion. Below the last lumbar vertebra is a wedge-shaped bone which has the appearance of five vertebra fused together. This bone, known as the sacrum, connects with the large bones which form the pelvic girdle. Attached to the lower end of the sacrum is a group of from two to four small vertebra, more or less fused, called the coccyx.

The Joining of the Vertebra. - A typical vertebra consists of a heavy, disk-shaped portion in front, called the body, which is connected with a ring-like portion behind, called the neural arch. The body and the neural arch together encircle a round opening which is a part of the canal that contains the spinal cord. From the neural arch are seven bony projections, or processes, three of which serve for the attachment of muscles and ligaments, while the other four, two above and two below, are for the interlocking of the vertebra with each other. The separate vertebra are joined together in the spinal column, as follows:

a. Between the bodies of adjacent vertebra are disks of elastic cartilage. Each disk is about one fourth of an inch thick and is grown tight onto the face of the vertebra above and also onto the face of the vertebra below. By means of these disks a very close connection is secured between the vertebra on the front side of the column.

b. On the back of the column, the downward projections from the neural arch of each vertebra above fit into depressions found in the neural arch of the vertebra below. This interlocking of the vertebra, which is most marked in the lumbar region, strengthens greatly the back portion of the column.

c. To further secure one bone upon the other, numerous ligaments pass from vertebra to vertebra on all sides of the column.

2. The Skull. - The skull is formed by the close union of twenty-two irregular bones. These fall naturally into two subgroups - the cranium and the face. The cranium consists of eight thin, curved bones which enclose the space, called the cranial cavity, that holds the brain. The face group, consisting of fourteen bones, provides cavities and supports for the different organs of the face, and supplies a movable part (the inferior maxillary) which, with the bones above (superior maxillary), forms the machine for masticating the food.

3. The Thorax. - This group contains twenty-four bones of similar form, called ribs, and a straight flat bone, called the sternum, or breastbone. The ribs connect with the spinal column behind, and all but the two lowest ones connect with the sternum in front, and, by so doing, enclose the thoracic cavity. As already stated, the bones of the thorax form a mechanical device, or machine, for breathing. The ribs are so arranged that the volume of the thorax is increased by elevating them and diminished by depressing them, enabling the air to be forced into and out of the lungs.

4. The Shoulder and Pelvic Girdles. - These groups form two bony supports - one at the upper and the other at the lower portion of the trunk - which serve for the attachment of the arms and legs. The shoulder girdle is formed by four bones - two clavicles, or collar bones, and two scapula, or shoulder blades. The clavicle on either side connects with the upper end of the sternum and serves as a brace for the shoulder, while the scapula forms a socket for the humerus (the large bone of the arm) and supplies many places for the attachment of muscles.

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