THE STRUCTURAL BASTS OF THE BODY 37 



body, can procure a still more perfect uniformity in the chemical 

 conditions to which every cell is exposed. It is not till much later 

 that the organism achieves an independence of external conditions of 

 temperature. In the mammalia, by means of the reactive nervous 

 system, the heat produced in every vital activity by the chemical 

 changes of combustion and disintegration is so balanced against the 

 heat lost through the external surface to the environment that the 

 temperature of the internal fluid is maintained practically constant. 

 One of the main results of the differentiation of function and structure 

 is therefore a gradual setting free of the majority of the cells of the 

 body from the influence of variations in the environment ; and in the 

 highest type of all animals, in man, this independence of external con- 

 ditions is carried to a much further extent by conscious adaptations, 

 such as the use of clothes, dwellings, artificial heating, and so on. 



The differentiation of the cells which compose the organs of the 

 body is determined in the first place by the different conditions to 

 which they are exposed in virtue of their positions in the course of 

 development. All the higher animals may be considered as built in 

 the form of a tube, the external surface of which is modified for the 

 purpose of defence and for adaptation to changes in the environ- 

 ment. From this layer there are developed not only the protective 

 cuticle, but also the organs of motor reaction, namely, the special 

 senses and the nervous system. The internal surface of the tube 

 is modified for purposes of alimentation. From it are developed all 

 those structures which serve for the digestion of the food-stuffs, for 

 their absorption into the common circulating fluid, for their elabora- 

 tion after absorption, and their preparation for utilisation by other 

 cells of the body. Between these two surfaces are situated the support- 

 ing tissues of the body as well as the organs for the conversion of 

 the potential energy of the body into motion and work, namely, the 

 muscles. Here also is the coelom or body cavity, represented in the 

 higher animals by the pleural and peritoneal cavities. The alimentary 

 canal projects for a considerable part of its course into this ccelom, 

 being attached to the body wall only by one side. From the coelom 

 is also developed the blood vascular system, surrounded by contractile 

 and connective cells which maintain a constant circulation of the 

 blood throughout the body. By this differentiation the body becomes 

 divided into a number of organs, each of which is composed of like cells, 

 modified for a common function and bound together by connective 

 tissue, the latter serving also to carry the blood-vessels which convey 

 the common medium for the working cells. In the study of physiology 

 our task consists, firstly, in the description of the special part taken 

 by each organ in the general functions of the body, and, secondly, in 

 the determination of the limiting conditions of such functions and of the 



