PHYSIOLOGY OF THE CIRCULATION. 81 



tending force produce, in each, totally distinct 

 phenomena. 



The coats of the large arteries being dense, thick, 

 and highly distensible, it is evident that an accumu- 

 lation of blood in, and a forcible distension of, those 

 elastic tubes, are the only effects which can occur 

 from the detention within them of a quantity of 

 highly compressed blood ; for the escape of any 

 portion of the contained fluid through the pores of 

 the investing tissues is prevented by the great thick- 

 ness and dense structure of the arterial tunics. It 

 is also quite clear that any considerable accumula- 

 tion of fluid can only take place in a tube when the 

 walls of that tube are either flaccid or elastic ; and 

 that, in the latter case, the amount of the accumu- 

 lation will, to a certain extent, be regulated by the 

 distending power of the contained fluid. Since, 

 therefore, various functions have been traced to the 

 reaction of the elastic coats of the large arteries upon 

 the distending force of the blood accumulated within 

 them, and since that distending force has been shown 

 to be no other than the lateral pressure of the arterial 

 blood, we are, I conceive, now authorised to advance 

 a step further, and say, that the functions in question 

 are referrible to that lateral pressure as to the cause 

 essentially producing them. The uses to which I 

 more particularly allude are the equal distribution 

 of the heart's impulse throughout the system, and 

 the uniform and equable flow of blood through the 

 capillaries and veins. 



The properties of the smaller arteries are so very 

 different from those of the vessels we have just been 



G 



