The peculiar structure and form of the skull afford a perfect shelter for the brain—an organ so delicate that, if unprotected, an ordinary blow would destroy it. Its oval or egg shape adapts it to resist pressure. The smaller and stronger end is in front, where the danger is greatest. Projections before and behind shield the less protected parts. The hard plates are not easy to penetrate. [Footnote: Instances have been known where bullets, striking against the skull, have glanced off, been flattened, or even split into halves. In the Peninsular Campaign, the author saw a man who had been struck in the forehead by a bullet which, instead of penetrating the brain, had followed the skull around to the back of the head, and there passed out.] The spongy packing deadens every blow. [Footnote: An experiment resembling the familiar one of the balls in Natural Philosophy ("Steele's Popular Physics," Fig. 6, p. 26), beautifully illustrates this point. Several balls of ivory are suspended by cords, as in Fig. 5. If A be raised and then let fall, it will transmit the force to B, and that to C, and so on until F is reached, which will fly off with the impulse. If now a ball of spongy bone be substituted for an ivory one anywhere in the line, the force will be checked, and the last ball will not stir.] The separate pieces with their curious joinings disperse any jar which one may receive, and also prevent fractures from spreading.
FIG. 5.
[Illustration]
The frequent openings in this strong bone box afford safe avenues for the passage of numerous nerves and vessels which communicate between the brain and the rest of the body.
FIG. 6.
[Illustration: The Spine; the seven vertebræ of the neck, cervical; the twelve of the back, dorsal; the five of the loins, lumbar; a, the sacrum, and b, the coccyx, coming the nine "false vertebræ." (p. 3).]
2 THE TRUNK.
THE TRUNK has two important cavities. The upper part, or chest, contains the heart and the lungs, and the lower part, or abdomen, holds the stomach, liver, kidneys, and other organs (Fig. 31). The principal bones are those of the spine, the ribs, and the hips.
THE SPINE consists of twenty-four bones, between which are placed pads of cartilage. [Footnote: These pads vary in thickness from one fourth to one half an inch. They become condensed by the weight they bear during the day, so that we are somewhat shorter at evening than in the morning. Their elasticity causes them to resume their usual size during the night, or when we lie down for a time.] A canal is hollowed out of the column for the safe passage of the spinal cord. (See Fig. 50.) Projections (processes) at the back and on either side are abundant for the attachment of the muscles. The packing acts as a cushion to prevent any jar from reaching the brain when we jump or run, while the double curve of the spine also tends to disperse the force of a fall. Thus on every side the utmost caution is taken to guard that precious gem in its casket.
THE PERFECTION OF THE SPINE surpasses all human contrivances. Its various uses seem a bundle of contradictions. A chain of twenty-four bones is made so stiff that it will bear a heavy burden, and so flexible that it will bend like rubber; yet, all the while, it transmits no shock, and even hides a delicate nerve within that would thrill with the slightest touch. Resting upon it, the brain is borne without a tremor; and, clinging to it, the vital organs are carried without fear of harm.