55. But though by the analysis of its component parts, this machine, so complex in its construction, and so wonderfully endowed, may be reduced to this state of simplicity; and although this analytical view of it be highly useful in enabling us to form a clear conception of the nature of its composition; yet it is only by considering its individual parts such as they actually are, and by studying their situation, connexion, structure, and action, that we can understand it as a whole, and apply our knowledge of it to any practical use. 56. Viewing then the human body as a complicated whole, as a congeries of organs made up of various combinations of simple tissues, it may be observed, in reference to its external configuration, that it is rounded. This rounded form is principally owing to the large proportion of fluids which enter into its composition. The roundness of the face, limbs, and entire surface of the child, are in striking contrast to the unequal and irregular surface of the old man, whose humours are comparatively very much smaller in quantity. 57. The length of the human body exceeds its breadth and thickness; the degree of the excess varying at different periods of life, and according to the peculiar constitution of the individual. In the extremities, the bones, muscles, vessels, and nerves, are especially distinguished by their length. [Illustration: Fig. XXXIV. Front view of the skeleton. 1. the head; 2. the trunk; 3. the superior extremities; 4. the inferior extremities.] [Illustration: Fig. XXXV. 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).