THE NATURE AND THE MEANS OF DIGESTION 31 



example, when meat is boiled the common variety of con- 

 nective tissue in it is converted to gelatin. This change 

 is a typical hydrolysis, and if it were not executed in ad- 

 vance it would be an early task of the gastric juice. When 

 cooking is attended by considerable drying of the food it is 

 less likely to count definitely toward digestion. Most 

 proteins are coagulated by heat, and this change to solid 

 form seems opposed to the general course of events in the 

 alimentary system. The action of microscopic organisms 

 upon food substances is in line more or less with normal 

 digestion; the maturing of cheese is an example. But 

 bacterial action may depart so far from the normal de- 

 composition as to generate products of strongly poisonous 

 properties, the so-called ptomains being among them. 



The Means of Digestion. Hydrolytic cleavages closely 

 resembling those of animal digestion may be caused to 

 occur in various ways. Boiling food-stuffs with acids ac- 

 complishes this. So does treatment with alkalis. Similar 

 results follow the application of superheated steam. But 

 the striking fact is that such changes as are brought about 

 in the laboratory by violent reagents, high temperatures, 

 or both in conjunction, are caused to take place in the 

 stomach and the intestine by bland juices acting at the 

 mild temperature of the body. The changes effected by 

 these juices are often modified by the simultaneous 

 activity of bacteria, but the presence of the latter is to be 

 regarded as accidental and non-essential. 



The power to digest foods has been known for a long 

 time to reside in the secretions which enter the alimentary 

 tract. It was at first necessarily estimated simply by 

 observing the progressive solution of solid food. The 

 intimate nature of the process has become appreciated 

 more recently. Comparison of the juices from different 

 sources shows that they are individual and specific to the 

 extent that each one, as a rule, acts upon certain classes 

 of food and not on all. There is sufficient evidence for 

 the belief that when a juice digests two or more classes 

 of food-stuffs it contains separate and distinct reagents 



