136 INFECTION AND RESISTANCE 



creased intracellular pressure. This, in turn, brings about the ex- 

 trusion from the cell of proteins and other ordinarily non-diffusible 

 substances, and destruction of the cell results. This explanation is 

 practically an adaptation of the earlier more primitive osmotic the- 

 ories to the facts subsequently discovered. It stands in direct con- 

 tradiction to the prevailing opinion that the process of bacteriolysis 

 and cytolysis in general is an enzymotic process, brought about by 

 the injury of the cell by specific substances comparable to digestive 

 ferments. Interesting though the suggestion of Baumgarten is, it 

 can hardly receive more than casual attention given it for the sake 

 of completeness, since careful experimental work by von Lingel- 

 sheim 8 has shown definitely that altered salt contents of serum 

 do not exercise the effect upon bacteriolysis which we would be en- 

 titled to expect from Baumgarten' s reasoning. 



In explanation of the natural immunity possessed by many ani- 

 mals against various infections, Baumgarten has offered another 

 explanation which, like the preceding, we may classify, in agreement 

 with Sauerbeck, 9 with the "passive" theories. This theory, which 

 he calls his "Assimilation Theory,' 7 assumes that the bacteria do not 

 find suitable food material in the tissues and fluids of certain ani- 

 mals, and, since bacteria do not have to be killed to be eliminated, 

 but may be checked merely by their inability to grow and multiply, 

 they must soon succumb in surroundings in which they find no suit- 

 able foodstuffs. This point of view approaches somewhat the earlier 

 exhaustion theory of Pasteur, which has been mentioned in another 

 place. 10 



In contrast to these "Passive" theories of immunity are the now 

 prevailing and well-founded opinions that the resistance of the ani- 

 mal body against bacterial invasion is not a mere fortuitous result 

 of chemical and physical conditions encountered by the infectious 

 agents, but is rather the result of the struggle against the invasion 

 by active forces of the body cells and fluids. The part played by the 

 cells had already been emphasized by Metchnikoff and his school 

 when the discovery of the bactericidal power of the normal blood 

 was made. The study of the antibacterial powers of the blood now 

 introduced a new element which became the basis of the so-called 

 "humoral" theories. In the prolonged controversies w T aged, with 

 great astuteness and experimental skill, between the adherents of 

 these two schools, most of the facts which we possess regarding im- 

 munity were discovered, and it is only within recent years that we 



8 Von Linerelsheim. Zeitschr. f. Hyg., Vol. 37, 1901. 



Sauerbeck. "Die Krise in der Immumtatsforschung," Klinkhardt, 

 Leipzisr, 1009. 



10 The influence of foodstuffs, temperature, and other environmental con- 

 ditions upon natural immunity has been discussed in an earlier section. 



