362 



DIGESTION 



When trypsin ads upon protein already digested by pepsin, this 

 partially hydrolysed residue is smaller than when the trypsin acts 

 alone, no matter for how long a time. Also the decomposition of 

 a given quantity of protein by trypsin is accomplished in a notably 

 shorter time if it has been previously subjected to the action of 

 pepsin. This illustrates the co-operative relation of these two 

 ferments a relation still more clearly implied in the fact that, 

 although trypsin readily forms albumoses and peptones from native 

 protein when such is offered to it, yet in natural digestion the great 

 albumose- and peptone-forming ferment is pepsin. In the lumen of 

 the intestine the trypsin is confronted mainly with protein already 

 hydrolysed to the albumose and peptone stage in the stomach. In 

 other words, instead of the very large molecules of the original 

 protein food with a weight of perhaps 5,000 to 7,000, the trypsin 

 begins its action on a much larger number of much smaller molecules 

 of only one-twentieth the initial weight or less. The statement is 

 sometimes made that trypsin is a stronger proteolytic ferment than 

 pepsin. This may be true in the sense that trypsin carries the de- 

 composition down to bodies of smaller molecular weight than pepsin. 

 But within the range of its hydrolytic action pepsin decomposes 

 certain proteins and allied bodies more readily than trypsin e.g., 

 the serum proteins, and especially elastin and the constituents of 

 connective tissue. 



In all that we have hitherto said regarding tryptic digestion we 

 have supposed that putrefaction has been entirely prevented e.g., 

 by the addition of toluol. If no antiseptic is added to a tryptic 

 digest, it rapidly becomes filled with micro-organisms, and emits a 

 very disagreeable faecal odour ; and now various bodies which are not 

 found in the absence of putrefaction make their appearance, such as 

 indol, skatol, and other substances, to which the fascal odour is due. 

 They are not true products of tryptic digestion, but are formed by 

 the putrefactive micro-organisms, which can themselves split off 

 from proteins numerous decomposition products, including tyrosin, 

 and change tyrosin into indol. 



Amylase or pancreatic amylase, the diastatic or sugar- forming 

 ferment of pancreatic juice, changes starch into dextrin and maltose, 

 just as the ptyalin of saliva does. The two ferments are possibly 

 identical, but under the conditions of action of the pancreatic juice 

 its diastatic power is greater than that of saliva, and it readily acts 



