216 PEACTICAL PHYSIOLOGY 



and B. typhosus, on the other hand, which do not excrete their 

 ferment, but retain it in their cell bodies as a zymase. The bacilli- 

 free culture medium, when injected into an animal, does not in this 

 case produce any symptoms. 



The unorganised ferments which exist in the animal body are 

 divided into certain groups depending on their action. Some convert 

 polysaccharides into monosaccharides. These are called amylolytic 

 ferments, examples being ptyalin in saliva and amylopsin in pancreatic 

 juice. Others convert native proteids into peptones. These are called 

 proteolytic, and are found in the gastric (pepsin) and pancreatic juices 

 (trypsin). Others invert disaccharides, and are called inversive e.g. 

 invertin of intestinal juice. Others split neutral fat into fatty acid 

 and glycerine, and are called steatolytic e.g. steapsin of pancreatic 

 juice ; whilst others convert soluble into insoluble proteids e. g. the 

 rennet of the gastric juice converts caseinogen into casein, and are 

 called coagulative. Fibrin and myosin ferments have a similar 

 action on certain of the soluble proteids of blood and muscle. 



These ferments all act best at the temperature of the body. They 

 become inactive at low temperatures, but this does not destroy them, 

 as they again reassume activity on raising the temperature. As stated 

 above, a temperature of 70 C. destroys them if water be present. If 

 they are dried, however, they can stand much higher temperatures. 

 Some of them act best in an alkaline reaction (ptyalin) ; others in 

 an acid reaction (pepsin). 



The substances produced by their activity tend to stop their action, 

 e.g. the alcohol produced by the action of yeast on cane sugar will, if 

 allowed to accumulate, ultimately put a stop to the fermentation. 

 In the case of organised ferments these products may actually kill the 

 cell, and completely stop any further fermentation. In the case of 

 unorganised ferments, on the other hand, if these substances be removed, 

 the ferment resumes activity. 



Most ferments, o.erobic, require free oxygen for their activity, others, 

 anaerobic, can act in the absence of the free gas. 



The ferments cannot be isolated as tangible substances, but they can 

 be precipitated along with proteids by saturation with ammonia sul- 

 phate, or by the addition of alcohol. They are soluble in glycerine, and 

 active solutions of them are usually prepared by extracting the gland 

 with this substance. There are no chemical tests by which we can 

 identify them, their presence in any fluid being detected by allowing 

 them to act on suitable substances. 



Nearly all ferments act by producing hydrolysis, e.g. the inversion 

 of cane sugar, the peptic or tryptic digestion of proteids, etc. 



