12 THE MECHANISTIC THEORY OF LIFE 



where very rapid control is required that the 

 nervous system plays an important part. 



It is not merely in the case of functional 

 activity that chemical stimuli play an im 

 portant part: for evidence is now steadily 

 accumulating that the co-ordination of growth 

 and maintenance in different parts of the body 

 is dependent on the action of chemical sub 

 stances conveyed from part to part by the 

 blood and lymph circulation, or by simple 

 diffusion from one contiguous part to another. 

 Some of the most striking evidence in this 

 direction has been afforded by the extraor 

 dinary effect on growth and nutrition which is 

 produced by removal or disease of such organs 

 as the thyroid gland or the pituitary body. 

 Absence of the thyroid completely arrests 

 growth, and influences in other ways the 

 normal structure of the body; but these effects 

 can be annulled by the administration of an 

 extract of the thyroid from another animal. 

 Hence even such a very small organ as the 

 thyroid produces chemical substances which 

 excite and control the nutrition of other parts. 

 Another well-known case is the influence 

 of changes in the generative organs on the 



