The palate bones are behind those of the upper jaw and with them form the bony part of the roof of the mouth. The inferior turbinated are spongy, scroll-like bones, which curve about within the nasal cavities so as to increase the surface of the air passages of the nose.
The vomer serves as a thin and delicate partition between the two cavities of the nose. It is so named from its resemblance to a ploughshare.
Fig. 15.--The Base of the Skull.
A, palate process of upper jawbone;
B, zygoma, forming zygomatic arch;
C, condyle for forming articulation with atlas;
D, foramen magnum;
E, occipital bone.
The longest bone in the face is the inferior maxillary, or lower jaw. It has a horseshoe shape, and supports the lower set of teeth. It is the only movable bone of the head, having a vertical and lateral motion by means of a hinge joint with a part of the temporal bone.
35. Sutures of the Skull. Before leaving the head we must notice the peculiar and admirable manner in which the edges of the bones of the outer shell of the skull are joined together. These edges of the bones resemble the teeth of a saw. In adult life these tooth-like edges fit into each other and grow together, suggesting the dovetailed joints used by the cabinet-maker. When united these serrated edges look almost as if sewed together; hence their name, sutures. This manner of union gives unity and strength to the skull.
In infants, the corners of the parietal bones do not yet meet, and the throbbing of the brain may be seen and felt under these "soft spots," or fontanelles, as they are called. Hence a slight blow to a babe's head may cause serious injury to the brain (Fig. 14).
The Bones of the Trunk.
36. The Trunk. The trunk is that central part of the body which supports the head and the upper pair of limbs. It divides itself into an upper cavity, the thorax, or chest; and a lower cavity, the abdomen. These two cavities are separated by a movable, muscular partition called the diaphragm, or midriff (Figs. 9 and 49).
The bones of the trunk are variously related to each other, and some of them become united during adult life into bony masses which at earlier periods are quite distinct. For example, the sacrum is in early life made up of five distinct bones which later unite into one.
The upper cavity, or chest, is a bony enclosure formed by the breastbone, the ribs, and the spine. It contains the heart and the lungs (Fig. 86).
The lower cavity, or abdomen, holds the stomach, liver, intestines, spleen, kidneys, and some other organs (Fig. 59).
The bones of the trunk may be subdivided into those of the spine, the ribs, and the hips.
The trunk includes 54 bones usually thus arranged:
1 Spinal Column, 26 bones:7 Cervical Vertebræ.12 Dorsal Vertebræ.5 Lumbar Vertebræ.1 Sacrum.1 Coccyx.
2 Ribs, 24 bones:14 True Ribs.6 False Ribs.4 Floating Ribs.
3 Sternum.
4 IV. Two Hip Bones.
5 V. Hyoid Bone.
37. The Spinal Column. The spinal column, or backbone, is a marvelous piece of mechanism, combining offices which nothing short of perfection in adaptation and arrangement could enable it to perform. It is the central structure to which all the other parts of the skeleton are adapted. It consists of numerous separate bones, called vertebræ. The seven upper ones belong to the neck, and are called cervical vertebræ. The next twelve are the dorsal vertebræ; these belong to the back and support the ribs. The remaining five belong to the loins, and are called lumbar vertebræ. On looking at the diagram of the backbone (Fig. 9) it will be seen that the vertebræ increase in size and strength downward, because of the greater burden they have to bear, thus clearly indicating that an erect position is the one natural to man.
Fig. 16.--The Spinal Column.
This column supports the head, encloses and protects the spinal cord, and forms the basis for the attachment of many muscles, especially those which maintain the body in an erect position. Each vertebra has an opening through its center, and the separate bones so rest, one upon another, that these openings form a continuous canal from the head to the lower part of the spine. The great nerve, known as the spinal cord, extends from the cranium through the entire length of this canal. All along the spinal column, and between each two adjoining bones, are openings on each side, through which nerves pass out to be distributed to various parts of the body.
Between the vertebræ are pads or cushions of cartilage. These act as "buffers," and serve to give the spine strength and elasticity and to prevent friction of one bone on another. Each vertebra consists of a body, the solid central portion, and a number of projections called processes. Those which spring from the posterior of each arch are the spinous processes. In the dorsal region they are plainly seen and felt in thin persons.
The bones of the spinal column are arranged in three slight and graceful curves. These curves not only give beauty and strength to the bony framework of the body, but also assist in the formation of cavities for important internal organs. This arrangement of elastic pads between the vertebræ supplies the spine with so many elastic springs, which serve to break the effect of shock to the brain and the spinal cord from any sudden jar or injury.
The spinal column rests on a strong three-sided bone called the sacrum, or sacred-bone, which is wedged in between the hip bones and forms the keystone of the pelvis. Joined to the lower end of the sacrum is the coccyx, or cuckoo-bone, a tapering series of little bones.
Experiment 7. Run the tips of the fingers briskly down the backbone, and the spines of the vertebræ will be tipped with red so that they can be readily counted. Have the model lean forward with the arms folded across the chest; this will make the spines of the vertebræ more prominent.
Experiment 8. To illustrate the movement of torsion in the spine, or its rotation round its own axis. Sit upright, with the back and shoulders well applied against the back of a chair. Note that the head and neck can be turned as far as 60° or 70°. Now bend forwards, so as to let the dorsal and lumbar vertebræ come into play, and the head can be turned 30° more.
Experiment 9. To show how the spinal vertebræ make a firm but flexible column. Take 24 hard rubber overcoat buttons, or the same number of two-cent pieces, and pile them on top of each other. A thin layer of soft putty may be put between the coins to represent the pads of cartilage between the vertebræ. The most striking features of the spinal column may be illustrated by this simple apparatus.
38. How the Head and Spine are Joined together. The head rests upon the spinal column in a manner worthy of special notice. This consists in the peculiar structure of the first two cervical vertebræ, known as the axis and atlas. The atlas is named after the fabled giant who supported the earth on his shoulders. This vertebra consists of a ring of bone, having two cup-like sockets into which fit two bony projections arising on either side of the great opening (foramen magnum) in the occipital bone. The hinge joint thus formed allows the head to nod forward, while ligaments prevent it from moving too far.
On