bones of the _fore arm_; _x_, _x_, those of the _wrists_; _y_, _y_, those of the _fingers_; _z_, _z_, are the round heads of the thigh bones, where they are inserted into the sockets of the bones of the pelvis, giving motion in every direction, and forming the _hip joint_; a b, a b, are the _thigh bones_; c, c, the _knee joints_; d e, d e, the _leg bones_; f, f, the _ankle joints_; g, g, the _bones of the foot_. 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 vertebrae; viz. the _cervical_, (from the Latin, _cervix_, the neck,) the _dorsal_, (from _dorsum_, the back,) and _lumbar_, (from _lumbus_, the loins.) [Illustration: Fig. 2.] Fig. 2, represents one of the _cervical vertebrae_. Seven of these, placed one above another, constitute that part of the spine which is in the neck. [Illustration: Fig. 3.] Fig. 3, is one of the _dorsal vertebrae_, twelve of which, form the central part of the spine. [Illustration: Fig. 4.] Fig. 4, represents one of the _lumbar vertebrae_, (five in number,) which are immediately above the sacrum. These vertebrae 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 vertebrae 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 vertebrae; 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 vertebrae, 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.