GENERAL STRUCTURE AND COMPOSITION. 1 



The Limbs. The limbs present no such arrangement of 

 cavities on each side of a bony axis as is seen in the trunk. 

 They have an axis formed at different parts of one or more 

 bones (as seen at U 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, how- 



u 



FIG. 4. A section across the forearm a short distance below the elbow-joint. R 

 and 7, 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 nuin 

 bers ; n n. nerves and vessels. 



ever, 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 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 Chem- 

 ical Anatomy. But as long as the Body is alive it is the seat 

 of constant chemical transformations in its material, and 

 these arc inseparably connected with its functions, the great 

 majority of which are in the long-run dependent upon chem- 

 ical changes. From this point of view, then, the chemical 

 study of the Body presents physiological problems, and it is 

 usual to include all the facts known as to the chemical com- 

 position and metamorphoses of living matter under the name 



