Hygienic Physiology | Page 7

in general, of two compact plates, with a
spongy layer between. These are in several pieces, the outer ones being
joined by notched edges, sutures (su'tyurs,), in the way carpenters term
dovetailing. (See Fig. 4.)
FIG. 4.
[Illustration: The Skull.--1. frontal bone; 2, parietal bone; 3, temporal
bone; 4, the sphenoid bone; 5, ethmoid bone; 6, superior maxillary
(upper jaw) bone; 7, malar bone; 8, _lachrymal bone; 9, nasal bone; 10,
inferior maxillary (lower jaw) bone._]
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.
FIG. 7.
[Illustration: B, the first cervical vertebra, the atlas; A, _the atlas, and
the second cervical vertebra, the axis; e, the odontoid process; c, the
foramen._]
THE SKULL ARTICULATES with (is jointed to) the spine in a
peculiar manner. On the top of the upper vertebra (atlas [Footnote:
Thus called because, as, in ancient fable, the god Atlas supported the
world on his shoulders, so in the body this bone bears the head.]) are
two little hollows (a, b, Fig. 7), nicely packed and lined with the
synovial membrane, into which fit the corresponding projections on the
lower part of the skull, and thus the head can rock