METABOLISM 



225 



concerned, then, are for the most part colloids, which do not readily 

 osmose. They therefore do not make their way out of the cell as fast as 

 formed, like the others, but collect in the cytoplasm, and either osmose 

 out of it slowly or pass off bodily in a mass. Among these substances 

 are proteids, fats, glycogen, mucin, and numerous other products of 

 anabolism, even less known than these, such as enzymes, pigments, and 

 "extractives" of many sorts and uses. Many of these products probably 

 have molecules so large and unstable (because complicated and loosely 

 composed within) that they are broken up into parts before passing out 

 of the cell. This is the case particularly with the granules which are 

 conspicuous in secretory processes of the first type (e. g., in salivary, 

 pancreatic, hepatic cells). In case of mucus, not diffusible through 

 membranes, the cell gives way bodily and allows the product to escape 

 freely into the lumen of the gland. 



FIG. 119 



C D 



Secreting epithelium (parotid of a rabbit), to show the process, so far as to be seen under the 

 microscope: A, resting, B, after action occasioned by pilocarpine; C, after more vigorous action 

 occasioned by both pilocarpine and stimulation of the sympathetic; D, after long-continued 

 action from stimulation of the sympathetic. (Langley.) 



THE INTERNAL SECRETIONS are poured into the blood instead of into 

 the gut or on the skin. Among those of chief importance so far dis- 

 covered are those produced by the pancreas, the duodenum, the liver, the 

 thyroid, the thymus, the pituitary body, the adrenals, the kidneys, the 

 spleen, the testes, and the ovaries. In addition to these, the coccygeals 

 and the carotids produce internal secretions of unknown value, while 

 all the lymphoid tissues secrete lymph. We can here mention only in a 

 most summary way the uses of these various products secreted into the 

 circulation. Little is as yet learned about them probably compared 

 with what some day will be known. 

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