Hygienic Physiology | Page 8

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 over and even cross the ulna.
THE WRIST, or carpus, consists of two rows of very irregular bones, one of which articulates with the forearm; the other, with the hand. They are placed side to side, and so firmly fastened as to admit of only a gliding motion. This gives little play, but great strength, elasticity, and power of resisting shocks.
THE HAND.--The metacarpal (meta, beyond; karpos, wrist), or bones of the palm, support each a thumb or a finger. Each finger has three bones, while the thumb has only two. The first bone of the thumb, standing apart from the rest, enjoys a special freedom of motion, and adds greatly to the usefulness of the hand.
FIG. 12.
[Illustration: Bones of the Hand and the Wrist.]
The first bone (Figs. 11, 12) of each finger is so attached to the corresponding metacarpal bone as to move in several directions upon it, but