310 SECRETION. 



when the activity of the secretion is very greatly diminished, as in the saliva. 

 These periods of repose seem to be necessary to the proper action of the 

 secreting glands ; forming a marked contrast with the constant action of 

 organs of excretion. It is well known, for example, that digestion is seri- 

 ously disturbed when the act is too prolonged on account of the habitual in- 

 gestion of an excessive quantity of food. 



From the considerations already mentioned, it is evident that the charac- 

 teristic constituents of the true secretions are formed by the epithelial 

 structures of the glands. While the mechanism of this process is not under- 

 stood in all its details as regards all of the secretions, in some of the glands 

 the processes have been studied with tolerably definite results. In some of 

 the salivary glands, in the peptic cells and in the cells of the pancreas, it 

 has been shown that the so-called ferments are not formed directly. 

 The secreting cells are apparently divided into two portions, or zones ; an 

 outer zone, which is next the tubular membrane, and an inner zone, next 

 the lumen of the tube or follicle. In the inner zone, during the intervals 

 of actual secretion, there appears a substance, which at the time when the 

 secretion is formed and is poured out, is changed into the true ferment, 

 or active principle of the secretion ; so that there is probably a zymogenic, 

 or ferment-forming substance, first produced by the cells. The substance, 

 if such a substance exists, out of which ptyaline is formed, has not been 

 described ; but in the viscid forms of saliva, there appears to be first formed 

 a substance called mucinogen, afterward changed into mucine, upon which 

 the viscidity of the fluid depends. 



In the salivary glands which produce viscid secretions, the submaxillary 

 and sublingual, the parenchyma presents two kinds of acini, serous and 

 mucous. The so-called serous acini are the more abundant and are thought 

 to produce the true saliva, while the mucous acini secrete the viscid con- 

 stituents of the saliva. 



In the production of pepsine, the inner zone of the peptic cells first 

 forms pepsinogen, which is changed into pepsine as it is discharged from 

 the glands. In the pancreas, trypsinogen is formed in the inner zone of the 

 cells, and this is changed into trypsine. The general name zymogen has 

 been given to the substances which are changed into the digestive ferments ; 

 although, as is evident, this substance is not identical in the different glands. 

 The formation of the .ferments of the true secretions is analogous in its na- 

 ture to certain of the nutritive processes. 



The theory that the discharge of the secretions is due simply to mechan- 

 ical causes and is attributable solely to the increase in the pressure of blood 

 can not be sustained. Pressure undoubtedly has considerable influence upon 

 the activity of secretion ; but the flow will not always take place in obedi- 

 ence to simple pressure, and secretion may be excited for a limited time 

 without any increase in the quantity of blood circulating in the gland. 



The glands possess a peculiar excitability, which is manifested by their 

 action in response to proper stimulation. During secretion, they generally 

 receive an increased quantity of blood ; but this is not indispensable, and 



