RESPIRATION 35 



although, as we have seen, acetic acid is a common by- 

 product in anaerobic fermentations. Acetic acid is produced 

 on a commercial scale either by destructive distillation of 

 wood, or by the slow oxidation of dilute solutions of alco- 

 hol in the presence of free oxygen (L e. in the air) by living 

 organisms. The optimum temperature for these (20-30 C.) 

 is lower than the optimum for alcoholic (25-30 C.), butyric 

 (35 C.), and lactic (45-50 C.) fermentations, and they 

 can withstand a decidedly higher amount (8-14%) of free 

 acid than most other organisms.* So long as the acetic 

 bacteria are adequately supplied with alcohol for respira- 

 tion, they continue to form acetic acid to the amount above 

 indicated ; but if the supply of alcohol is insufficient, they 

 oxidize some of the acetic acid, setting free carbon-dioxide, 

 which is not produced under favorable conditions. 



Besides these fermentations, which are carried on in solu- 

 tions by specific organisms, there are similar processes car- 

 ried on in the living body by specific parasitic organisms, 

 mainly bacteria, which liberate the energy and supply 

 themselves with the food which they need by attacking 

 either the living substance of the body itself or the lifeless 

 food-substances enclosed within its cells. The main and the 

 by-products of these parasitic organisms cause the system- 

 atic disturbances of the functions of the host organism, 

 which are characteristic of the different forms of disease. 

 So, for instance, the bacillus of diphtheria (Bacillus diph- 

 therite, K-L.) causes decompositions in the limited area of 

 the mucous membrane of the throat, which it commonly 

 attacks, and the products of these decompositions, poisons, 

 diffusing through the body of the host, are the direct causes 

 of the disease rather than the presence, merely locally irri- 

 tating, of the parasitic organisms. This discovery, that the 

 poisonous products of the specific bacteria of disease, and 

 not the bacteria themselves, are the direct causes of disease, 

 has opened a new chapter in the science of medicine ; but so 

 long as our knowledge of the chemistry of these products, 



* Citromyces, according to Wehmer (Beitrage z. Kenntnis einheimischer 

 Pilze. I), can withstand successfully at least 20% of the citric acid which 

 it forms from sugars. 



