354 A MANUAL OF PHYSIOLOGY 



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 prozymogens. 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 prozymo- 

 gen, the precursor of the zymogen or pepsinogen, as pepsinogen 

 is the precursor of the enzyme pepsin. 



A glycerin or watery extract of the salivary glands always 

 contains 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 dis- 

 posed 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. 415, 416). 



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 filters, but living laboratories, acquires 

 additional strength. It is evident that everything in the secre- 

 tion 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 



