288 THE HUMAN BODY. 



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 

 outer medium. 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 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 removal 

 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 chemical 

 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 solu- 

 tion causes the rearrangement into carbon dioxide, alcohol, 

 glycerine and succinic acid, of many atoms of carbon, hydro- 

 gen and oxygen which previously existed as sugar; and 

 a very considerable quantity of sugar may be broken up by 

 the activity of a few living yeast-cells. How the latter act 

 we do not know with certainty, but most likely by picking cer- 

 tain atoms out of the sugar molecule, and leaving the rest to 

 fall down into simpler compounds. On the other hand, we find 

 cells which form and store up in themselves large quantities 

 of substances, which they afterwards liberate; starch, for 

 instance, being formed and laid by in many fruit-cells, and 



