31. The Head, or Skull. The bones of the skeleton, the bony framework of our bodies, may be divided into those of the head, the trunk, and the limbs.

The bones of the head are described in two parts,--those of the cranium, or brain-case, and those of the face. Taken together, they form the skull. The head is usually said to contain 22 bones, of which 8 belong to the cranium and 14 to the face. In early childhood, the bones of the head are separate to allow the brain to expand; but as we grow older they gradually unite, the better to protect the delicate brain tissue.

32. The Cranium. The cranium is a dome-like structure, made up in the adult of 8 distinct bones firmly locked together. These bones are:

One Frontal, Two Parietal, Two Temporal One Occipital, One Sphenoid, One Ethmoid.

The frontal bone forms the forehead and front of the head. It is united with the two parietal bones behind, and extends over the forehead to make the roofs of the sockets of the eyes. It is this bone which, in many races of man, gives a dignity of person and a beauty of form seen in no other animal.

The parietal bones form the sides and roof of the skull. They are bounded anteriorly by the frontal bone, posteriorly by the occipital, and laterally by the temporal and sphenoid bones. The two bones make a beautiful arch to aid in the protection of the brain.

The temporal bones, forming the temples on either side, are attached to the sphenoid bone in front, the parietals above, and the occipital behind. In each temporal bone is the cavity containing the organs of hearing. These bones are so called because the hair usually first turns gray over them.

The occipital bone forms the lower part of the base of the skull, as well as the back of the head. It is a broad, curved bone, and rests on the topmost vertebra (atlas) of the backbone; its lower part is pierced by a large oval opening called the _foramen magnum_, through which the spinal cord pa.s.ses from the brain (Fig. 15).

The sphenoid bone is in front of the occipital, forming a part of the base of the skull. It is wedged between the bones of the face and those of the cranium, and locks together fourteen different bones. It bears a remarkable resemblance to a bat with extended wings, and forms a series of girders to the arches of the cranium.

The ethmoid bone is situated between the bones of the cranium and those of the face, just at the root of the nose. It forms a part of the floor of the cranium. It is a delicate, spongy bone, and is so called because it is perforated with numerous holes like a sieve, through which the nerves of smell pa.s.s from the brain to the nose.

[Ill.u.s.tration: Fig. 14.--The Skull]

33. The Face. The bones of the face serve, to a marked extent, in giving form and expression to the human countenance. Upon these bones depend, in a measure, the build of the forehead, the shape of the chin, the size of the eyes, the prominence of the cheeks, the contour of the nose, and other marks which are reflected in the beauty or ugliness of the face.

The face is made up of fourteen bones which, with the exception of the lower jaw, are, like those of the cranium, closely interlocked with each other. By this union these bones help form a number of cavities which contain most important and vital organs. The two deep, cup-like sockets, called the orbits, contain the organs of sight. In the cavities of the nose is located the sense of smell, while the buccal cavity, or mouth, is the site of the sense of taste, and plays besides an important part in the first act of digestion and in the function of speech.

The bones of the face are:

Two Superior Maxillary, Two Malar, Two Nasal, Two Lachrymal, Two Palate, Two Turbinated, One Vomer, One Lower Maxillary.

34. Bones of the Face. The superior maxillary or upper jawbones form a part of the roof of the mouth and the entire floor of the orbits.

In them is fixed the upper set of teeth.

The malar or cheek bones are joined to the upper jawbones, and help form the sockets of the eyes. They send an arch backwards to join the temporal bones. These bones are remarkably thick and strong, and are specially adapted to resist the injury to which this part of the face is exposed.

The nasal or nose bones are two very small bones between the eye sockets, which form the bridge of the nose. Very near these bones are the two small lachrymal bones. These are placed in the inner angles of the orbit, and in them are grooves in which lie the ducts through which the tears flow from the eyes to the nose.

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 pa.s.sages of the nose.

The vomer serves as a thin and delicate part.i.tion between the two cavities of the nose. It is so named from its resemblance to a ploughshare.

[Ill.u.s.tration: 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 sh.e.l.l 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 part.i.tion 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 ma.s.ses 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:

I. Spinal Column, 26 bones: 7 Cervical Vertebrae.

12 Dorsal Vertebrae.

5 Lumbar Vertebrae.

1 Sacrum.

1 Coccyx.

II. Ribs, 24 bones: 14 True Ribs.

6 False Ribs.

4 Floating Ribs.

III. Sternum.

IV. Two Hip Bones.

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 vertebrae. The seven upper ones belong to the neck, and are called cervical vertebrae. The next twelve are the dorsal vertebrae; these belong to the back and support the ribs. The remaining five belong to the loins, and are called lumbar vertebrae. On looking at the diagram of the backbone (Fig. 9) it will be seen that the vertebrae 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.

[Ill.u.s.tration: 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 ca.n.a.l 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 ca.n.a.l. All along the spinal column, and between each two adjoining bones, are openings on each side, through which nerves pa.s.s out to be distributed to various parts of the body.

Between the vertebrae 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 a.s.sist in the formation of cavities for important internal organs. This arrangement of elastic pads between the vertebrae 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.