THE ACTIVITIES OF THE ORGANISM 97 



is carried away by factors of the jugular vein. Two 

 nerves supply the gland : one is the chorda tympani, 

 a branch of a cranial nerve, and the other is a sym- 

 pathetic nerve. Lymph also leaves the gland by a 

 little vessel. 



Now suppose we have laid bare all this mechanism 

 in a living animal and make experiments upon it. 

 If we stimulate the chorda tympani there is a copious 

 flow of thin watery saliva, but if we stimulate the 

 sympathetic there is a less copious flow of thick viscid 

 saliva. Why is this ? We find on closer analysis 

 that the chorda contains fibres which dilate the small 

 arteries so that there is an increased flow of blood 

 through the gland ; but that, on the other hand, the 

 sympathetic contains fibres which constrict the 

 arteries, thus leading to a reduced flow of blood. This 

 accounts for the fact that " chorda-saliva " is abundant 

 and thin, while " sympathetic-saliva " is scarce and 

 thick. It was thought at one time that the chorda 

 contained fibres which stimulated the gland to produce 

 watery saliva, while the sympathetic contained fibres 

 which stimulated it to produce mucid saliva. This, 

 however, is not the case. Both nerves contain the 

 same kind of secretory fibres : their other fibres differ 

 mainly in that they act differently on the arteries. 



It might be the case — indeed it was at one time 

 thought that it was the case — that secretion of saliva 

 was simply a matter of blood-flow : an abundant 

 arterial circulation gave rise to abundant saliva, a 

 sparse flow to a sparse saliva. Undoubtedly the 

 secretion depends on blood supply, but not solely. 

 If it did, then the whole process might be conceived to 

 be a very simple mechanical one — filtration or diffusion 

 of the saliva from the blood stream through the thin 

 walls of the blood vessels, and the walls of the tubules 



G 



