8 THE HUMAN BODY. 



tively, are not however characteristic of the limbs, for they 

 are present in abundance in the dorsal and ventral cavities 

 and in their walls. 



FIG. 4. A section across the forearm a short distance below the elbo w- joint. 

 R and U, its two supporting bones, the radius and ulna : e, the epidermis, and d 

 the dermis of the skin; the latter is continuous below with bands of connective 

 tissue, s, which penetrate between and invest the muscles, which are indicated 

 by numbers; n, n, nerves and vessels. 



Chemical Composition of the Body. In addition to the 

 study of the Body as composed of tissues and organs which 

 are optically recognizable, we may consider it as composed 

 of a number of different chemical substances. This branch 

 of knowledge, which is still very incomplete, really presents 

 two classes of problems. On the one hand we may limit 

 ourselves to the examination of the chemical substances 

 which exist in or may be derived from the dead Body, or, 

 if such a thing were possible, from the living Body entirely 

 at rest; such a study is essentially one of structure and 

 may be called Chemical Anatomy. But as long as the 

 Body is alive it is the seat of constant chemical trans- 

 formations in its material, and these are inseparably con- 

 nected with its functions, the great majority of which are 

 in the long-run dependent upon chemical changes. From 

 this point of view, then, the chemical study of the Body 

 presents physiological problems, and it is usual to include 

 all the known facts as to the chemical composition and 

 metamorphoses of living matter under the name of Physio- 

 logical Chemistry. For the present we may confine our- 

 selves to the more important substances derived from or 

 known to exist in the Body, leaving questions concerning the 

 chemical changes taking place within it for consideration 

 along with those functions which are performed in connec- 

 tion 'with them. 



