298 PHYSIOLOGY CHAP. 



alcoholism, infection (syphilis), or toxic products developed by the 

 bacteria found in the intestines. He lays stress upon the enormous 

 number of bacteria which accumulate in the large intestine, since 

 the remains of undigested food afford an excellent culture medium 

 for microbes. Bacteria indeed form a large proportion of the faeces. 

 The question whether the bacteria found in the intestines serve a 

 useful purpose in the process of digestion is one which has given 

 rise to much discussion and to many experiments. Metschnikoff 

 regards them not merely as useless but as actually harmful. In 

 support of this theory he adduces the fact that in diseases in which 

 there is intestinal stasis the toxic products become diffused 

 throughout the organism, as was proved by Ewald in the case of 

 a patient suffering from an intestinal fistula ; these toxic products 

 suffice in the absence of any other cause to set up arteriosclerosis 

 and hence senility. Metschnikoff made further experimental 

 researches into old age in 1910, and showed that aortic atheroma, 

 cirrhosis of the liver, and chronic interstitial nephritis may be 

 induced in animals by small daily doses of paracresol continued 

 for months at a time. Okhuba, and after him Bratchinsky (1912), 

 obtained organic lesions like those found in the organs of old 

 people by administering minute doses of indol to rabbits, guinea- 

 pigs, and monkeys. 



Eibbert, however, observed that this theory of intoxication 

 rests entirely upon hypothetical grounds. The idea that it is 

 possible for bacterial toxins to become reabsorbed in the normal 

 healthy intestine under normal conditions of life amounts to the 

 admission of the possibility of physiological intoxication, a con- 

 tradiction in terms. Such poisonings do not take place, because 

 under normal conditions all the more or less toxic products of 

 metabolism which are developed in the organism are promptly 

 eliminated by it. 



Bibber t observes that the retrogressive changes of old age 

 take place in all the organs and in the same typical order in each 

 of them, resulting in the state of physiological senility. It is 

 therefore arbitrary to assert that enterogenic toxins must act 

 mainly on the vascular system, and that the changes in this system 

 bring about all the other changes, even those in the brain, and 

 are thus responsible for death from old age. 



Metschnikoff is of the opinion that the most specialised and 

 most active elements of the organism succumb to the action 

 of those poisons, and the lower connective and supporting 

 tissues grow at their expense. In order to explain this pheno- 

 menon of substitution which is identified with the process of 

 growing old, he has recourse to the doctrine of phagocytosis. The 

 macrophages in the circulation have no difficulty in coming into 

 contact with every part of the organism, and normally do good 

 defensive service by absorbing abnormal or extraneous elements, 



