THE LIVING ORGANISM 229 



uniting system of arteries, capillaries, and 

 veins, the life-maintaining fluid is diverted 

 into thousands or even millions of capillary 

 channels, in which it is only separated from 

 the fluid bathing the needy tissues, by thin, 

 transparent, permeable cells compared to the 

 thinness of which the finest tissue paper is 

 as thick as hide. Here the commerce of 

 chemical exchange goes on in a never-closing 

 market. Day or night, asleep or awake, 

 nutriment is taken in and poisonous excretory 

 substances given out ; but also all types 

 of beautifully manufactured chemical goods 

 are exchanged. It must not be supposed 

 that nutriment alone is taken and waste 

 returned, in exchange for some single obvious 

 service to the community, such as a muscular 

 contraction, a nervous stimulus given at the 

 proper instant of time, or the elaboration of 

 a digestive secretion. 



The classes of cells are greater artists, and 

 take wider interests than this ; few of them 

 resemble in narrowness of life and austerity 

 of outlook the so-called human specialist. 

 In nearly every case the cell of a given type 

 possesses what might perhaps be described 

 as a hobby in addition to its more obvious 

 function, and the secret hobby is often 

 more all-important than the obvious day's 



