274 THE TISSUES AND MECHANISMS OF DIGESTION. 



"granules" during secretion, should stir up the cell to an increased activ- 

 ity in forming granules, and that the formative activity should cease 

 when the secretory activity ceased. In such a case the number of new 

 granules formed might always be equal to the number of old granules 

 used up, and the active cell, in spite of its discharge, would possess as 

 many granules that is to say, as large a load as the cell at rest. And 

 in the central gastric cells of some animals it would appear that such a 

 continued balancing of load and discharge does actually take place, so 

 that no distinction in granules can be observed between resting and 

 active cells. 



208. We spoke just now of the material stored up in the cell and 

 destined to form part of the secretion as undergoing change before it was 

 discharged. In the mucous cell we have seen that the material deposited in 

 the living cell has at first the form of granules. These granules, however, 

 are easily converted into a transparent material lodged in the spaces of the 

 cell-substance, which material, even if not exactly identical with, at least 

 closely resembles, the mucin found in the secretion ; and apparently in the 

 act of secretion the granules do undergo some such change. In the case 

 of some other glands, moreover, we have chemical as well as optical 

 evidence that the material stored up in the cells is, in part at least, not 

 the actual substance appearing in the secretion, but an antecedent of that 

 substance. 



An important constituent of pancreatic juice is, as we shall see later on, 

 a body called trypsin, a ferment very similar to pepsin, acting on proteid 

 bodies and converting them into peptone and other substances. Though in 

 many respects alike, pepsin and trypsin are quite distinct bodies, and differ 

 markedly in this, that while an acid medium is necessary for the action of 

 pepsin, an alkaline medium is necessary for the action of trypsin ; and accord- 

 ingly the pancreatic juice is alkaline in contrast to the acidity of gastric juice. 

 Trypsin can, like pepsin ( 191), be extracted with glycerin from substances 

 in which it occurs ; glycerin extracts of trypsin, however, need for the 

 manifestation of their powers the presence of a weak alkali, such as a 1 

 per cent, solution of sodium carbonate. 



Now, trypsin is present in abundance in normal pancreatic juice ; but a 

 loaded pancreas, one which is ripe for secretion, and which if excited to 

 secrete would immediately pour out a juice rich in trypsin, contains no 

 trypsin or a mere trace of it ; nay, even a pancreas which is engaged in the 

 act of secreting contains in its actual cells an insignificant quantity only of 

 trypsin, as is shown by the following experiment: 



If the pancreas of an animal, even of one in full digestion, be treated, 

 while still warm from the body, with glycerin, the glycerin extract, as judged 

 of by its action on fibrin in the presence of sodium carbonate, is inert or nearly 

 so as regards proteid bodies. If, however, the same pancreas be kept for 

 twenty-four hours before being treated with glycerin, the glycerin extract 

 readily digests fibrin and other proteids in the presence of an alkali. If the 

 pancreas, while still warm, be rubbed up in a mortar for a few minutes with 

 dilute acetic acid, and then treated with glycerin, the glycerin extract is 

 strongly proteolytic. If the glycerin extract, obtained without acid from the 

 warm pancreas, and therefore inert, be diluted largely with water and kept 

 at 35 C. for some time, it becomes active. If treated with acidulated instead 

 of distilled water, its activity is much sooner developed. If the inert glycerin 

 extract of warm pancreas be precipitated with alcohol in excess, the precipi- 

 tate, inert as a proteolytic ferment when fresh, becomes active when exposed 

 for some time in an aqueous solution, rapidly so when treated with acidulated 

 water. These facts show that a pancreas taken fresh from the body, even 



