Hygienic Physiology | Page 8

to and fro. The
second vertebra (axis) has a peg, e, which projects through a hole, c, in
the first.
FIG. 8.
[Illustration: The Thorax or Chest. a, the sternum; b to c, _the true ribs;
d to h, the false ribs; g, h, the floating ribs;_ i, k, the dorsal vertebræ.]
The surfaces of both vertebræ are so smooth that they easily glide on
each other, and thus, when we move the head side wise, the atlas turns
around the peg, e, of the axis.
THE RIBS, also twenty-four in number, are arranged in pairs on each
side of the chest. At the back, they are all attached to the spine. In front,
the upper seven pairs are tied by cartilages to the breastbone (sternum);

three are fastened to each other and to the cartilage above, and two, the
floating ribs, are loose.
The natural form of the chest is that of a cone diminishing upward. But,
owing to the tightness of the clothing commonly worn, the reverse is
often the case. The long, slender ribs give lightness, [Footnote: If the
chest wall were in one bone thick enough to resist a blow, it would be
unwieldy and heavy As it is, the separate bones bound by cartilages
yield gradually, and diffuse the force among them all, and so are rarely
broken.] the arched form confers strength, and the cartilages impart
elasticity,--properties essential to the protection of the delicate organs
within, and to freedom of motion in respiration. (See note, p. 80.)
FIG. 9.
[Illustration: The Pelvis. a, the sacrum; b, b, _the right and the left
innominatum._]
THE HIP BONES, called by anatomists the innominata, or nameless
bones, form an irregular basin styled the pelvis (pelvis, a basin). In the
upper part, is the foot of the spinal column--a wedge-shaped bone
termed the sacrum [Footnote: So called because it was anciently
offered in sacrifice.] (sacred), firmly planted here between the
widespreading and solid bones of the pelvis, like the keystone to an
arch, and giving a steady support to the heavy burden above.
3. THE LIMBS.
TWO SETS OF LIMBS branch from the trunk, viz.: the upper, and the
lower. They closely resemble each other. The arm corresponds to the
thigh; the forearm, to the leg; the wrist, to the ankle; the fingers, to the
toes. The fingers and the toes are so much alike that they receive the
same name, digits, while the several bones of both have also the
common appellation, phalanges. The differences which exist grow out
of their varying uses. The foot is characterized by strength; the hand, by
mobility.
FIG. 10.

[Illustration: The Shoulder Joint. a, the clavicle; b, the scapula.]
1. THE UPPER LIMBS.--THE SHOULDER.--The bones of the
shoulder are the collar bone (clavicle), and the shoulder blade (scapula).
The clavicle (clavis, a key) is a long, slender bone, shaped like the
Italic f. It is fastened at one end to the breastbone and the first rib, and,
at the other, to the shoulder blade. (See Fig. 1.) It thus holds the
shoulder joint out from the chest, and gives the arm greater play. If it be
removed or broken, the head of the arm bone will fall, and the motions
of the arm be greatly restricted. [Footnote: Animals which use the
forelegs only for support (as the horse, ox, etc.), do not possess this
bone. "It is found in those that dig, fly, climb and seize."]
THE SHOULDER BLADE is a thin, flat, triangular bone, fitted to the
top and back of the chest, and designed to give a foundation for the
muscles of the shoulder.
THE SHOULDER JOINT.--The arm bone, or humerus, articulates with
the shoulder blade by a ball-and-socket joint. This consists of a cup-like
cavity in the latter bone, and a rounded head in the former, to fit it,--
thus affording a free rotary motion. The shallowness of the socket
accounts for the frequent dislocation of this joint, but a deeper one
would diminish the easy swing of the arm.
FIG. 11.
[Illustration: Bones of the right Forearm. H, the humerus; R, the radius;
and U, the ulna.]
THE ELBOW.--At the elbow, the humerus articulates with the ulna--a
slender bone on the inner side of the forearm--by a hinge joint which
admits of motion in only two directions, i. e., backward and forward.
The ulna is small at its lower end; the radius, or large bone of the
forearm, on the contrary, is small at its upper end, while it is large at its
lower end, where it forms the wrist joint. At the elbow, the head of the
radius is convex and fits into a shallow cavity in the ulna, while at the
wrist the ulna plays in a similar socket in the radius. Thus the radius
may roll