404 THE CHEMICAL COMPOSITION OF THE BODY. [CH. xxv. 



poisonous, but which are useful as foods. The most virulent 

 poison in existence, namely snake poison, is a proteid of the 

 proteose class. 



There is another class of chemical transformations which differ 

 very considerably from all of these. They, however, resemble 

 these fermentations in the fact that they occur independently of 

 any apparent change in the agents that produce them. The 

 agents that produce them are not living organisms, but chemical 

 substances, the result of the activity of living cells. The 

 change of starch into sugar by the ptyalin of the saliva is an 

 instance. 



Ferments may therefore be divided into two classes : 



1. The organised ferments torulae, bacteria, &c. 



2. The unorganised ferments, or enzymes like ptyulin. 



ob 00 



f 



Fig. 339. Types of micro-organisms, a, micrococci arranged singly ; in twos, diplococci 

 if all the micrococci at a were grouped together they would be called staphylococci 

 and in fours, sarcinte ; 6, micrococci in chains, streptococci ; c and d, bacilli of various 

 kinds (one is represented with flagellum) ; e, various forms of spirilla ; /, spores, either 

 free or in bacilli. 



Each may be again subdivided according to the nature of the 

 chemical change produced. 



In digestion, the study of which we shall soon be commencing, 

 it is the unorganised ferments with the action of which we have 

 chiefly to deal. 



The unorganised ferments may be classified as follows : 



(a) Amylolytic those which change amyloses (starch, glyco- 

 gen) into sugars. Examples : ptyalin, diastase, amylopsin. 



(b) Proteolytic those which change proteids into proteoses 

 and peptones. Examples : pepsin, trypsin. 



(c) Steatolytic those which split fats into fatty acids and 

 glycerin. An example, steapsin, is found in pancreatic juice. 



(d) Inversive those which convert saccharoses (cane sugar, 

 maltose, lactose) into glucose. Examples : invertin of intestinal 

 juice and of yeast cells. 



