94 CONSTITUTION OF PROTOPLASM 



that the production of antitoxin may be increased by an injection 

 of pilocarpin. 



If we reject this explanation of the phenomenon, and regard 

 the production of antitoxin as being analogous to the process of 

 internal secretion, we must regard it as being due to the action of 

 the living cells when they are stimulated by the toxin. There is 

 at the present time but one theory which suggests how this may 

 be brought about : this is Ehrlich's celebrated side-chain theory, 

 which has played so important a part in the modern study of 

 problems concerning infection and immunity, and which, though 

 highly hypothetical, deserves a close and careful study. 



Ehrlich points out that a cell has two functions. The first 

 is the physiological function, which it enacts in the body secretory 

 in the case of a gland cell, conductive in the case of a nerve fibre, 

 etc. ; and the second is the function of nutrition, necessary to 

 supply the waste which is constantly taking place. We must 

 suppose a similar constitution for each of the complex molecules 

 of living protoplasm which build up the cell ; in each molecule 

 there is a portion which discharges the specific function of the 

 cell, and this portion has to be nourished. The functional portion 

 is the more important, and is called by Ehrlich the active centre 

 (Leistungskern). In our diagrams we shall represent it as forming 

 the central portion of the giant molecule, but it is hardly necessary 

 to caution the reader that this diagrammatic representation has 

 no necessary morphological basis. It is just as permissible to 

 regard this portion as diffused through the nutritive part of the 

 molecule as long as the difference in constitution and action of the 

 two are borne in mind. 



The second portion, that concerned in nutrition, is more 

 important in relation to immunity. We may regard it as having 

 two functions it " seizes " suitable molecules of food substances 

 from the blood or lymph in which it is bathed, and it alters them 

 in such a way as to build them up into living material which can 

 take its place in the molecule of protoplasm, so as to replace that 

 which has been used in the life of the cell. 



The function of " seizing " molecules of food from the sur- 

 rounding tissues implies a selective faculty, for we cannot imagine 

 that all the molecules which circulate in the blood and lymph are 

 suitable for all cells at all times. This apparently intelligent 

 selection of suitable material is, of course, at bottom a chemical 

 process : the food molecule becomes attached to some portion of 



