PHYSIOLOGY OF THE CIRCULATION. 



89 



and as that increase or diminution in the relative 

 size of different portions of the blood-vessels of an 

 organ must necessarily be followed by corresponding 

 changes in the amount of lateral pressure acting on 

 the internal surface of those vessels : this doctrine is 

 in reality calculated to establish a still more intimate 

 connexion between the nervous system and these 

 functions, by showing how the former is enabled to 

 control the activity, and, at the same time, to modify 

 to a certain extent the character, of the latter. I 

 have now successively endeavoured to prove 



1. That the obstacles to the free passage of the 

 arterial blood prevent the small arteries from dis- 

 charging into the veins more than a limited quantity 

 of blood in a given time. 



2. That as the blood encounters fewer impedi- 

 ments in flowing into, than in escaping from, the 

 arterial system, its rate of influx has a constant 

 tendency to preponderate over its rate of efflux into 

 the veins. 



3. That the amount of this disproportion is ever 

 varying ; and that the evils which a great excess 

 would occasion are probably in some measure ob- 

 viated by the circumstance of the opposition to the 

 influx of blood into the arterial system increasing 

 in a direct ratio to the amount of disproportion 

 between it and the rate of the efflux of blood into 

 the veins. 



4. That the whole mass of blood contained in the 

 arterial system, from the heart to the most contracted 

 portion of the capillaries, exerts a lateral pressure, 

 the degree of which is altogether dependent upon 





