76 SENESCENCE AND REJUVENESCENCE 



relation between the narcotics and the cell lipoids. In the verte- 

 brates the accumulation and differentiation of lipoids, particularly 

 in the nervous system, is very much greater than in the lower inver- 

 tebrates, and it is probable that with some narcotics which are 

 highly fat soluble, the fundamental relation between susceptibility 

 and general metabolic condition is completely masked, or even 

 reversed, by their higher concentration in the cells of the nervous 

 system with a given external concentration, and consequently by 

 their greater narcotic effect on these cells. In the lower animals 

 and in early stages of development the action of narcotics is general, 

 but with the advance in differentiation the susceptibility of the 

 nervous system as compared with other organs increases very 

 greatly. In general it appears that the differences in susceptibihty 

 to all narcotics are much more nearly alike in the lower forms and 

 the early stages of all, while in the later stages of the higher forms 

 those substances which are highly water soluble act in much the 

 same way as in the lower forms, but the action of the highly fat- 

 soluble narcotics is modified because of the increasing development 

 and differentiation of Hpoids in the nervous system, and very 

 probably other modifications also occur. Nevertheless, and in 

 spite of these compHcating factors which appear in certain cases, 

 differences in susceptibility to various agents can, with proper 

 precautions and checks, be used to a certain extent as a means of 

 comparing general metabolic condition, even in the vertebrates. 

 The use of the cyanides seems to be freer from compHcating fac- 

 tors than that of other agents. 



Undoubtedly, however, the chief value of the susceptibility 

 method lies in its applicability to small simple organisms and to 

 different regions of a single, intact, not too highly differentiated 

 individual. By means of it we are able to gain some idea of differ- 

 ences in metabolic rate in many cases to which other methods are 

 not applicable. 



Thus far susceptibility to narcotics, cyanides, and other sub- 

 stances in its relation to metabolism has received but little atten- 

 tion. Lyon ('02, '04) and A. P. Mathews ('06) have used 

 susceptibility to cyanides and to various other substances and con- 

 ditions as a method for showing differences in rate of metaboHsm 



