Back view of the skeleton. 1. the head; 2. the trunk; 3. the superior extremities; 4. the inferior extremities.
58. The form of the human body is symmetrical, that is, it is capable of being divided into two lateral and corresponding halves. Suppose a median line to pass from the vertex of the head through the centre of the spinal column (fig. XXXIV. 1, 2); if the body be well formed, it will be divided by this line into two exactly equal and corresponding portions (fig. XXXV. 1). This symmetrical disposition of the body is not confined to its external configuration. It is true of many of the internal organs; but principally, as has been already stated, of those that belong to the animal life. The brain and the spinal cord are divisible into two exactly equal halves (figs. XLVIII. d, and XLIX. 1, 2, 3); the organs of sense are double and symmetrical: the muscles of one side of the body exactly correspond to those of the other (fig. XXXIII.); the two hands and arms and the two lower extremities are alike (figs. XXXIV., XXXV.); but for the most part, the organs of the organic life, the stomach, the intestines, the liver, the spleen, for example, are single, and not symmetrical.
59. The human body is divided into three great portions, the head, the trunk, and the extremities (figs. XXXIV. and XXXV. 1, 2, 3, 4).
60. By the head is meant all that part of the body which is placed above the first bone of the neck (fig. XXXIV. 1). It is of a spheroidal figure, broader and deeper behind than before, somewhat like an egg in shape, with the broad end behind; it is flattened at its sides (figs. XXXV. 1, and XXXVI. 2, 4). Its peculiar figure renders it at once stronger and more capacious than it could have been had it possessed any other form. It is supported by its base on the spinal column, to which it is attached by the peculiar structure termed a joint (fig. XXXIV.), and fastened by ligaments of exceeding strength.
61. The head contains the central organ of the nervous system; the organs of the senses, with the exception of that of touch; and the organs of mastication. It comprehends the cranium and the face. Both are composed partly of soft parts, as the teguments, namely, skin, fat, &c., and muscles; and partly of bones.
1. Frontal bone; 2. parietal bone; 3. occipital bone;
4. temporal bone; 5. nasal bone; 6. malar bone; 7. superior
maxillary bone; 8. inferior maxillary bone.
