180 



HABIT AND INTELLIGENCE. 



[chap. XV. 



Animal 



tissues. 



Possible 

 nervous 

 action in 

 increasing 

 nutrition 

 in exer- 

 cised parts. 



Increased 

 flow of 

 blood to 

 exercised 

 parts, 



possibly 

 due to re- 

 laxation 

 of the 

 nerves 

 of the 

 arteries. 



trunks and branches that are agitated by the wind. But it 

 ought not to be taken for granted that the case of muscular and 

 other animal tissue is parallel to this. In plants, the waste of 

 the tissues is very trifling, and it is probably null in vascular 

 tissue which is filling up and hardening into woody fibre ; so 

 that an increased flow of sap may very well fill up the vessels 

 with the substance it brings, just as drains are silted up. But 

 in animals, especially warm-blooded animals, the waste is great 

 and rapid ; and the more any organ is exercised, the more sub- 

 stance it loses by waste : and it is not easy to understand why 

 the increased flow of blood through an organ should not only 

 increase its nutrition (which it certainly will do), but cause the 

 nutrition to exceed the waste, so as to produce growth. Perhaps 

 the excess of nutrition to which the growth is due may be in 

 some way caused by nervous agency, which we know to be called 

 into play by every vital action whatever among those classes 

 of animals that have a well-developed nervous system. Such 

 action of the nervous system would, no doubt, be inexplicable ; 

 but it would not be more so than its action in stimulating 

 secretion,^ or, indeed, than any strictly vital action whatever. 



The cause of the increased flow of blood to and through parts 

 that are in exercise does not appear to be fully understood. 

 With respect to the muscles, I am inclined to think that Herbert 

 Spencer has assigned an adequate cause, namely, the varying 

 pressure on the blood-vessels during muscular action. But this, 

 obviously, will not apply to the flow of blood to the brain being 

 greater during the waking state than during sleep, or any other 

 flow that is directly produced by nervous action. Possibly the 

 increased flow of blood in this class of cases may be due to the 

 calibre of the small arteries being increased by the relaxation 

 of the nerves that control them ; but I only ofier this as a 

 suggestion. 



1 Carpenter's Human Phj'siology, p. 738. 



