Fig. [3], is one of the dorsal vertebræ, twelve of which, form the central part of the spine.
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.
