THE SECRETION OF THE DIGESTIVE JUICES 377 



when the trypsinogen has been activated into trypsin, or of the 

 gastric mucous membrane, when the pepsinogen has been changed 

 into pepsin, seems to be, roughly speaking, in proportion to the 

 quantity of granules present in the cells. Therefore it is concluded 

 that the granules represent mother-substances of the ferments or 

 zymogens. Some observers believe they have obtained evidence of 

 stages in the elaboration of the ferments still further back than the 

 mother-substances, grandmother-substances so to speak, or pro- 

 zymogens. Bensley, e.g., concludes that the nuclei of the chief cells 

 in the fundus glands of the stomach take part in the formation of a 

 prozymogen, the precursor of the zymogen or pepsinogen, as pepsino- 

 gen is the precursor of the enzyme pepsin. 



A glycerin or watery extract of the salivary glands always con- 

 tains active amylolytic ferment, if the natural secretion is active. 

 So that if ptyalin is preceded by a zymogen in the cells, it must be 

 very easily changed into the actual ferment. 



But we should greatly deceive ourselves if we supposed that granules 

 of this nature in gland-cells are necessarily related to the production of 

 ferments. The mucigenous granules have no such significance. Most 

 digestive secretions contain protein constituents, with which the 

 granules may have to do. as well as with ferments. And bile, a secretion 

 which contains no mucin, no proteins, and either no ferments or mere 

 traces, as essential and original constituents, is formed in cells with 

 granules so disposed and so affected by the activity of the gland as to 

 suggest some relation between them and the process of secretion. In 

 the liver-cells of the frog, in addition to glycogen, and oil-globules small 

 granules may be seen, especially near the lumen of the gland tubules; 

 they diminish in number during digestion, when the secretion of bile is 

 active and increase when food is withheld and secretion slow. And 

 in fasting dogs the secreting cells of Brunner's glands, the pyloric glands 

 and the pancreas, as well as the lining epithelium of the bile-ducts, 

 have been found to contain many fatty granules. Possibly some of 

 these represent the fat which is known to be excreted into the alimentary 

 canal (pp. 437, 438). 



The Nature of the Process by which the Digestive Secretions are 

 Formed. We have spoken more than once of the gland-cells as 

 manufacturing their secretions. It is an idea that rises naturally 

 in the mind as we follow with the microscope the traces of their 

 functional activity. And when we compare the composition of the 

 digestive juices with that of the blood- plasma and lymph, the 

 suggestion that the glands which produce them are not merely 

 passive niters, but living laboratories, acquires additional strength. 

 It is evident that everything in the secretion must, in some form 

 or other, exist in the blood which comes to the gland, and in the 

 lymph which bathes its cells. No glandular cell, if we except the 

 leucocytes, which in some respects are to be considered as unicellular 

 glands, dips directly into the blood ; everything a gland-cell receives 

 must pass through the walls of the bloodvessels. (But see footnote 



