GENERAL STRUCTURE AND COMPOSITION 7 



and R in Fig. 4, which represents a cross-section of the forearm 

 near the elbow-joint), but around this are closely-packed soft 

 parts, chiefly muscles, and the whole is enveloped in skin. The 

 only cavities in the limbs are branching tubes which are filled with 

 liquids during life, either blood or a watery-looking fluid known as 

 lymph. These tubes, the blood and lymph- 

 vessels respectively, are not, however, 

 characteristic of the limbs, for they are 

 present in abundance in the dorsal and 

 ventral cavities and in their walls. 



Chemical Composition of the Body. In 

 addition to the study of the Body as com- 

 posed of tissues and organs which are opti- n 



, , . , ., FIG. 4. A section across 



cally recognizable, we may Consider it as the forearm a short distance 



composed of a number of different chemi- ? d VYts *"$ 

 cal substances. This branch of knowl- bo : s > the radius and ulna; 



. . e, the epidermis, and a, the 



edge, which IS Still Very incomplete, really dermis of the skin; the latter 



i ,. -ii r\ ji is continuous below with 



presents two classes of problems. On the bands O f connective tissue, 

 one hand, we may limit ourselves to the anl^inVest^the 16 muscles 



examination of the chemical Substances which are indicated by num- 

 , . , . . . , , . , bers ; n, n, nerves and vessels. 



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 transformations in its 

 material, and these are inseparably connected 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 might be 

 called Chemical Physiology. At present it is customary to include 

 under the term Biological Chemistry the study of the chemical 

 structure of living matter and of the chemical changes occurring 

 in it. At this point we may confine ourselves to the more im- 

 portant 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 connection with them. 



Elements Composing the Body. Of the elements known to 

 chemists only seventeen have been found to take part in the 



