[Fig. 97]

Fig. 97-The human skeleton.

*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 _vertebrae_, and two bones found below the vertebrae, known as the sacrum and the coccyx (Fig.

98). These twenty-six bones supply the central axis of the body, support the head and upper extremities, and inclose and protect the spinal cord.

[Fig. 98]

Fig. 98-The spinal column.

The upper seven vertebrae form the neck and are called the _cervical_ vertebrae. They are smaller and have greater freedom of motion than the others. The first and second cervical vertebrae, 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 vertebrae, in order below the cervical, are known as the _thoracic_ vertebrae. They form the back part of the framework of the thorax and have little freedom of motion. The five vertebrae below the thoracic are known as the _lumbar_ vertebrae. 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 vertebrae 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 vertebrae, more or less fused, called the _coccyx_.

[Fig. 99]

Fig. 99-*Two views of a lumbar vertebra.* _A._ From above. _B._ From the side. 1. Body. 2, 3, 4, 5. Projections from the neural arch.

*The Joining of the Vertebrae.*-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 ca.n.a.l that contains the spinal cord (Fig. 99). 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 vertebrae with each other. The separate vertebrae are joined together in the spinal column, as follows:

_a._ Between the bodies of adjacent vertebrae 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 vertebrae 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 vertebrae, 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 pa.s.s 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 (Fig. 100). The _cranium_ consists of eight thin, curved bones which inclose the s.p.a.ce, 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.

[Fig. 100]

Fig. 100-*The skull (Huxley).* The ill.u.s.tration shows most of the bones of the skull.

3. _The Thorax._-This group contains twenty-four bones of similar form, called _ribs_, and a straight flat bone, called the _sternum_, or breastbone (Fig. 101). 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, inclose the thoracic cavity. As already stated (page 85), 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.

[Fig. 101]

Fig. 101-*Bone groups of trunk.*

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 (Fig. 101). The _shoulder girdle_ is formed by four bones-two clavicles, or collar bones, and two scapulae, 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.

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 (Fig. 102).

[Fig. 102]

Fig. 102-*Bone groups of arm and leg.*

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_ (Fig. 102).

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 the _carpus_, or 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 (Fig. 102).

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.

The different bone groups are shown in Fig. 97 and named in Table IV.

*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 particular 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 vertebrae 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 particular forms of service in the body.

TABLE IV - THE PRINc.i.p.aL BONES AND THEIR GROUPING IN THE BODY

I. AXIAL SKELETON

A. _Skull_, 28.

1. Cranium, 8.

_ a._ Frontal, forehead 1 _ b._ Parietal 2 _ c._ Temporal, temple 2 _ d._ Occipital 1 _ e._ Sphenoid 1 _ f._ Ethmoid 1

2. Face, 14.

_ a._ Inferior maxillary 1 _ b._ Superior maxillary 2 _ c._ Palatine, palate 2 _ d._ Nasal bones 2 _ e._ Vomer 1 _ f._ Inferior turbinated 2 _ g._ Lachrymal 2 _ h._ Malar, cheek bones 2

3. Bones of the Ears, 6.

_ a._ Malleus 2 _ b._ Incus 2 _ c._ Stapes 2

B. _Spinal Column_, 26.

1. Cervical, or neck, vertebrae 7 2. Dorsal, or thoracic, vertebrae 12 3. Lumbar vertebrae 5 4. Sacrum 1 5. Coccyx 1

C. _Thorax_, 25.