THE ACTIVITIES OF THE ORGANISM 99 



certain substances ; that these substances are then 

 washed out, so to speak, into the duct by the withdrawal 

 of water from the cell ; and that thereafter the cell 

 absorbs fresh nutritive material from the lymph which 

 exudes from the blood vessels, along with water. 

 The distinctive part of the whole train of processes is, 

 then, this elaboration of material by the cells them- 

 selves ; while the concomitant changes in the calibre 

 of the blood vessels and in the flow of blood and lymph 

 are subsidiary ones. In the process of secretion of 

 saliva energy is absorbed from the chemical substances 

 of the blood to bring about the passage of water from 

 a region of high to a region of low osmotic pressure ; 

 oxygen and nitrogen, with other elements of course, 

 are withdrawn from the arterial blood stream for the 

 purpose of the secretion, and carbon dioxide and other 

 substances are given off to the venous blood and 

 lymph. 



The problem thus is pushed back from the mechani- 

 cal events occurring in the nervous and circulatory pro- 

 cesses, to the physico-chemical ones occurring in the 

 cells of the gland tubules ; and it thus becomes much 

 more obscure. It is true that we can formulate a 

 hypothesis which describes, in a kind of way, these 

 intra-cellular metabolic changes, in terms of physico- 

 chemical reactions, and, without doubt, reactions of 

 this kind must occur within the cell. But if we could 

 test any such hypothesis as easily as the mechanical 

 ones suggested, should we find it any more self- 

 sufficient ? l 



1 We have not referred to " psychical secretion." If we smell some very 

 savoury substance our " mouth waters," that is, secretion of saliva occurs. 

 If we even see some such substance the same secretion occurs. All this is 

 clear and can be " explained " mechanistically : the stimulation of the 

 olfactory or visual organs begins a kind of reflex process. But if we even 

 think about some very savoury morsel saliva may be secreted. We must 



