272 SECRETION 



as regards all the secretions, in certain glands the processes have been 

 studied with fairly 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, or enzymes, are not formed directly. The secret- 

 ing cells are divided into two portions, or zones ; an outer zone, 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 is afterward changed into the true 

 ferment; so that there probably is a zymogenic, or ferment-forming 

 substance, first produced by the cells. It is thought that the substance 

 produced by the secreting cells of the salivary glands is not ptyalin but 

 a zymogen that is readily converted into ptyalin, called ptyalinogen; 

 but this substance has not been isolated. The parotid is classed as a 

 serous gland. In the viscid forms of saliva, there appears to be first 

 formed a substance called mucinogen, afterward changed into mucin, 

 on which the viscidity of the secretion depends. 



A good example of the changes which secreting cells undergo when 

 a resting gland becomes active may be observed in the pancreas. If the 

 pancreas is removed from a fasting animal and its structure is fixed and 

 stained, the cells lining the secreting alveoli are seen rather sharply 

 divided into the two zones already mentioned. The outer zone, next 

 the alveolar membrane, is narrow and stains deeply. The inner zone is 

 wide and granular and is but slightly stained. When the gland, how- 

 ever, is taken from an animal in full digestion, the outer zone occupies 

 the greater part of the cell, being deeply stained ; and the inner zone 

 has become very narrow. It seems, indeed, that the granular matter of 

 the inner zone is used in the production of the active principles of the 

 secretion. A short time after secretion has ceased, the gland will be 

 found to have returned to its resting condition and the granules of the 

 inner zone are restored. These processes take place in all glands and 

 mark their resting and active conditions. 



In the salivary glands that produce viscid secretions the submaxil- 

 lary 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 true saliva, while the mucous acini secrete only the 

 viscid matters that are mixed with the saliva. 



In the production of pepsin, the peptic cells first form pepsinogen, 

 which is afterward changed into pepsin. In the pancreas, trypsinogen 

 is formed in the cells, and this is changed into trypsin. The general 

 name zymogen has been given to the substances that are changed into 

 the digestive ferments ; although, as is evident, this is not identical in 

 the different glands. 



