438 THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



which are less saturated than oleic acid, and probably belong to the 

 linoleic series. It also yields a base, but it is doubtful if this is 

 identical with choline. Kephalin is the most abundant phosphatide 

 in nerve-fibres, and has also been found in egg-yolk. 



Sphingomyelin is the phosphatide obtained from the mixture 

 called protagon. It is the best known of the diamino-monophos- 

 phatides. If protagon is dissolved in hot pyridine, and the solution 

 allowed to cool, sphingomyelin is precipitated in an impure form as 

 sphaero-crystals, which rotate the plane of polarised light to the left. 

 Choline, fatty acids, and an alcohol have been found among its cleav- 

 age products. It, however, differs from lecithin by containing no 

 glycerin. 



Enzymes. 



The word fermentation was first applied to the change of sugar 

 into alcohol and carbonic acid by means of yeast. The evolution of 

 carbonic acid causes frothing and bubbling ; hence the term " fermen- 

 tation." The agent, yeast, which produces this, was called the ferment. 

 Microscopic investigation shows that yeast is composed of minute 

 rapidly-growing unicellular organisms belonging to the fungus group. 



The souring of milk, the transformation of urea into ammonium 

 carbonate in decomposing urine, and the formation of vinegar from 

 alcohol are brought about by very similar organisms. The complex 

 changes known as putrefaction, which are produced by the various 

 forms of bacteria (see fig. 316), also come into the same category. 



That the change or fermentation is produced by these organisms 

 is shown by the fact that it occurs only when the organisms are 

 present, and stops when they are removed or killed by a high 

 temperature or by antiseptics (carbolic acid, etc). 



The "germ theory" of disease explains the infectious diseases by 

 considering that the change in the system is of the nature of fermen- 

 tation, and, like the others we have mentioned, produced by microbes ; 

 the transference of the bacteria or their spores from one person to 

 another constitutes infection. The poisons produced by the growing 

 bacteria appear to be either alkaloid al (ptomaines) or protein in 

 nature. The most virulent poison in existence, namely, snake poison, 

 is a protein of the proteose class. 



All these micro-organisms require moisture in which to act. 

 They act best at a temperature of about 40 C. Their activity is 

 stopped, but the organisms are not destroyed by cold. The organisms 

 are, however, like other living cells, killed by too great heat. Some 

 micro-organisms act without free oxygen; these are called anaerobic ; 

 those that require oxygen are called aerobic. 



Another well-known fact concerning micro-organisms is that the 

 substances they produce in time put a stop to their activity ; thus 



