The bones are composed of two substances—one animal, and the other mineral. The animal part is a very fine network, called the cellular membrane. In this, are deposited the harder mineral substances, which are composed principally of carbonate and phosphate of lime. In very early life, the bones consist chiefly of the animal part, and are then soft and pliant. As the child advances in age, the bones grow harder, by the gradual deposition of the phosphate of lime, which is supplied by the food, and carried to the bones by the blood. In old age, the hardest material preponderates; making the bones more brittle than in earlier life.
As we shall soon have occasion to refer, particularly, to the spinal, or vertebral column, and the derangement to which it is liable, we give, on page 72, representations of the different classes of vertebræ; viz. the cervical, (from the Latin, cervix, the neck,) the dorsal, (from dorsum, the back,) and lumbar, (from lumbus, the loins.)
Fig. 2.
Fig. 2, represents one of the cervical vertebræ. Seven of these, placed one above another, constitute that part of the spine which is in the neck.
Fig. 3.
Fig. 3, is one of the dorsal vertebræ, twelve of which, form the central part of the spine.
Fig. 4.
Fig. 4, represents one of the lumbar vertebræ, (five in number,) which are immediately above the sacrum. These vertebræ are so fastened, that the spine can bend, in any direction; and the muscles of the trunk are used in holding it erect, or in varying its movements.
By the drawings here presented, it will be seen, that the vertebræ of the neck, back, and loins, differ somewhat in size and shape, although they all possess the same constituent parts; thus, A, in each, represents the body of the vertebræ; B, the articulating processes, by which each is joined to its fellow, above and below it; C, the spinous process, or that part of the vertebræ, which forms the ridge to be felt, on pressure, the whole length of the centre of the back. The back bone receives its name, spine, or spinal column, from these spinous processes.
It is the universal law of the human frame, that exercise is indispensable to the health of the several parts. Thus, if a blood-vessel be tied up, so as not to be used, it shrinks, and becomes a useless string; if a muscle be condemned to inaction, it shrinks in size, and diminishes in power; and thus it is also with the bones. Inactivity produces softness, debility, and unfitness for the functions they are designed to perform. This is one of the causes of the curvature of the spine, that common and pernicious defect in the females of America. From inactivity, the bones of the spine become soft and yielding; and then, if the person is often placed, for a length of time, in positions that throw the weight of the body unequally on certain portions of the spine, they yield to this frequent compression, and a distortion ensues. The positions taken by young persons, when learning to write or draw, or to play on the guitar, harp, or piano, and the position of the body when sleeping on one side, on high pillows, all tend to produce this effect, by throwing the weight of the body unequally, and for a length of time, on particular parts of the spine.
Fig. 5.
MUSCLES.
The muscles are the chief organs of motion, and consist of collections of fine fibres or strings, united in casings of membrane or thin skin. They possess an elastic power, like India rubber, which enables them to extend and contract. The red meat in animals consists of muscles. Every muscle has connected with it nerves, veins, and arteries; and those designed to move the bones, are fastened to them by tendons at their extremities. The muscles are laid over each other, and are separated by means of membranes and layers of fat, which enable them to move easily, without interfering with each other.
The figure on page 74, represents the muscles of the arm, as they appear when the skin and fat are removed. The muscles a and b are attached, at their upper ends, to the bone of the arm, and by their lower ends to the upper part of the fore arm, near the elbow joint. When the fibres of these muscles contract, the middle part of them grows larger, and the arm is bent at the elbow. The muscle c, is, in like manner, fastened, by its upper end, to the shoulder blade and the upper part of the arm, and by its lower end to one of the bones of the fore arm, near the elbow. When the arm is bent, and we wish to straighten it, it is done by contracting this muscle. The muscles d, d, are fastened at one end near the elbow joint, and at the other near the ends of the fingers; and on the back of the hand are reduced in size, appearing like strong cords. These cords are called tendons. They are employed in straightening the fingers, when the hand is shut. These tendons are confined by the ligament or band, e, which binds them down, around the wrist, and thus enables them to act more efficiently, and secures beauty of form to the limb. The muscles at f, are those which enable us to turn the hand and arm outward. Every different motion of the arm has one muscle to produce it, and another to restore the limb to its natural position. Those muscles which bend the body are called flexors; those which straighten it, extensors. When the arm is thrown up, one set of muscles is used; to pull it down, another set: when it is thrown forward, a still different set is used; when it is thrown back, another, different from the former; when the arm turns in its socket, still another set is used; and thus every different motion of the body is made by a different set of muscles. All these muscles are compactly and skilfully arranged, so as to work with perfect ease. Among them, run the arteries, veins, and nerves, which supply each muscle with blood and nervous power, as will be hereafter described. The size and strength of the muscles depend greatly on their frequent exercise. If left inactive, they grow thin and weak, instead of giving the plumpness to the figure, designed by Nature. The delicate and feeble appearance of many American women, is chiefly owing to the little use they make of their muscles. Many a pale, puny, shad-shaped girl, would have become a plump, rosy, well-formed person, if half the exercise, afforded to her brothers in the open air, had been secured to her, during childhood and youth.
NERVES.
The nerves are the organs of sensation. They enable us to see, hear, feel, taste, and smell; and also combine with the bones and muscles in producing motion.
The first engraving, on p. 77, (Fig. 6,) is a vertical section of the skull, and of the spinal column, or back bone, which supports the head, and through which runs the spinal cord, whence most of the nerves originate. It is a side view, and represents the head and spine, as they would appear, if they were cut through