called "voluntary" muscles, or the "muscles of animal life," while those out of the control of the will, are the "involuntary" muscles, or "muscles of organic life" (fig. 79). These last have a structure different to the voluntary muscles, being composed of flattened fibres containing granules and overlapping each other. The tendons are the cord-like extremities of the muscles which connect them to the bones; they are fibrous and slightly elastic, flexible, and exceedingly strong; while moist they have a splendent appearance, somewhat like mother-of-pearl. The ligaments are the strong fibrous bands which connect the bones together. Cartilages are substances of a white colour, smooth in their texture, and not very flexible; they are connected to the bones which they sometimes serve to prolong, and are themselves often converted into bone in old people, by the deposition of earthy matter within their structure, in fact, they may be considered as bone in an undeveloped state; for, in the infant, many parts are cartilaginous, which, in the adult, are bony; such as the ends of the long bones, the bones of the nose, &c. [Illustration: FIG. 79.--INVOLUNTARY MUSCULAR FIBRE.] [Illustration: FIG. 80.--HEAD OF AN INFANT.] [Illustration: FIG. 81.--HEAD OF AN ADULT.] With respect to the development of the form of the human body, there are two chief conditions which influence it, namely, age and sex. With respect to age, the proportional magnitudes of the different parts, early and late in life, vary very considerably. In the infant (fig. 80) the hand is by far the most completely developed (in size), next to this the abdomen, then the chest and upper extremities, and, finally, the lower extremities. In the adult (fig. 81), the largest measurement is round the chest and shoulders, but in the infant it is round the head; in the adult the lower extremities weigh one half of the whole body, but in the infant not one quarter. The infantile face has many peculiarities, so also has the face of old age (fig. 82). In the infant the lower part of the face is but little developed, the lower jaw is small, the chin scarcely at all prominent, and the distance from the nose to the chin very short, owing to the absence of the teeth, or (when formed) their smallness. The bones of the nose are scarcely formed, and this organ has therefore no bridge, properly so called; the nostrils are small, the lower part of the forehead small, smooth, and rounded, and the arches of the eyebrows but little prominent. The cheek-bones are small, and so are the bony arches which join them to the temples; the cheeks are full of fat, and the angles of the jaws rounded. In middle age the lower part of the forehead becomes more fully developed, the bony edges which support the eyebrows project and overhang the eyes, the bridge of the nose and the nostrils become more fully formed, the space from the nose to the chin is greater,