68 PHYSIOLOGY OF THE CIRCULATION. 



The first to be considered is the arborescent ar- 

 rangement of the arteries ; which not only increases 

 the surface of the vessels, and thus augments the 

 friction, but also implies the division of the main 

 current into as many minute streams as there are 

 arterial twigs. The impediment arising from this 

 cause must be materially augmented by the great 

 viscidity of the blood ; for the cohesion existing be- 

 tween its particles will oppose a considerable resist- 

 ance to their necessary separation from each other. 



2ndly. We have the obstacles to the flow of 

 blood occasioned by gravity the weight of adjacent 

 parts the friction consequent on the angular origin 

 and tortuous course of the arterial branches mus- 

 cular contraction the resistance of the blood pre- 

 viously contained in the vessels, and some of minor 

 importance. 



Srdly. The arterial blood, before entering the 

 capillaries, has to encounter the impediment to its 

 passage caused by the contractility of the smaller 

 arteries and capillaries. In mentioning this vital 

 property of the minute vessels as a chief obstacle to 

 the progress of the arterial blood, I would beg to re- 

 mark 1st, that our estimate of the aggregate 

 amount of resistance opposed to the blood's passage 

 through the small arteries is to be drawn from an 

 examination of that encountered by the separate 

 streams ; and not from a vague calculation as to the re. 

 lative areas of the aorta, and of its terminal branches ; 

 and 2ndly, thatas the contractility of the small arteries 

 and capillaries can, during its action, only cause a 

 diminution in the area of the tubes which they form, 



