CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 349 



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 

 accordingly the pancreatic juice is alkaline in contrast to the acid- 

 ity of gastric juice. Trypsin, can, like pepsin ( 183), be extracted 

 with glycerine from substances in which it occurs; glycerine ex- 

 tracts of trypsin however need for the manifestation of their 

 powers the presence of a weak alkali, such as a 1 p.c. 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 

 shewn 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 glycerine, the glyce- 

 rine extract, as judged of by its action on fibrin in the presence of 

 sodium carbonate, is inert or nearly so as regards proteid bdies. 

 If, however, the same pancreas be kept for 24 hours before being 

 treated with glycerine, the glycerine 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 glycerine, the 

 glycerine extract is strongly proteolytic. If the glycerine extract 

 obtained without acid from the warm pancreas, and therefore inert, 

 be diluted largely with water, and kept at 35C. for some time, 

 it becomes active. If treated with acidulated instead of distilled 

 water, its activity is much sooner developed. If the inert glyce- 

 rine extract of warm pancreas be precipitated with alcohol in 

 excess, the precipitate, inert as a proteolytic ferment when fresh, 

 becomes active when exposed for some time in an aqueous solu- 

 tion, rapidly so when treated with acidulated water. These facts 

 shew that a pancreas taken fresh from the body, even during full 

 digestion, contains but little ready-made ferment, though there is 

 present in it a body which, by some kinds of decomposition, giret 

 birth to the ferment. We may remark incidentally that though 

 the presence of an alkali is essential to the proteolytic action of 

 the actual ferment, the formation of the ferment out of its fore- 

 runner is favoured by the presence of a small quantity of neid ; the 

 acid must be used with care, since the trypsin, once formed, is 

 destroyed by acids. To this body, this mother of the ferment, 

 which has not at present been satisfactorily isolated, but which 



