THE SPECIFIC ELEMENTS OF SECRETIONS. 265 



the composition of a secretion cannot be accounted for in 

 this way; it contains some specific element, either a substance 

 which does not exist in the blood at all and must therefore 

 have been added by the secreting membrane, or some 

 body which, although existing in the blood, does so in such 

 minute proportion compared with that in which it is found 

 in the secretion, that some special activity of the secreting 

 cells is indicated; some affinity in them for these bodies by 

 which they actively pick them up. 



Each living cell, we have seen, is the seat of constant 

 chemical activity, taking up materials from the medium 

 about it, transforming and utilizing them, and sooner or 

 later restoring their elements, differently combined, to the 

 medium again. By such means it builds up and maintains 

 its living substance, and obtains energy to carry on its daily 

 work. While this is true of all cells in the Body, we find 

 certain groups in which chemical metabolism is the promi- 

 nent fact; cells which are specialized for this purpose just 

 as muscular fibre is for contraction or a nerve-fibre for con- 

 duction, and certain of these prominently metabolic tissues, 

 exist in the true glands and produce or collect the specific 

 elements of their secretions. Their chemical processes are 

 no doubt primarily directed to their own nutritive mainte- 

 nance; they live primarily for themselves, but their nutritive 

 processes are such that the bodies formed in them and sent 

 into the secretion are such as to be useful to the rest of the 

 cells of the community; or the bodies which they specially 

 collect, and in a certain sense feed on, are those the re- 

 moval of which from the blood is essential for the general 

 good. Their individual nutritive peculiarities are utilized 

 for the welfare of the whole Body. 



The Mode of Activity of Secretory Cells. If we con- 

 sider the modes of activity of living cells in general, it be- 

 comes clear that secretory cells may produce the specific 

 element of a secretion in either of two ways. They may, 

 as a by-result of their living play of forces, produce chemi- 

 cal changes in the surrounding medium; or they may build 

 up certain substances in themselves and then set them free 

 as specific elements. Yeast, for example, in a saccharine 



