116 PHYSICAL PHENOMENA OF CIRCULATION. [BOOK i, 



I. THE PHYSICAL PHENOMENA OF THE CIRCULATION. 



The apparatus concerned in the Maintenance of the Normal 

 Flow is composed of the following factors : 



1. The heart, beating rhythmically by virtue of its contractility 

 and intrinsic mechanisms, and at each beat discharging a certain 

 quantity of blood into the aorta. [For simplicity's sake we omit 

 for the present the pulmonary circulation.] 



2. The arteries, highly elastic throughout, with a circular mus- 

 cular element increasing in relative importance as the arteries 

 diminish in size. It must not be forgotten that the muscular 

 element is also elastic. 



When an artery divides, the united sectional area of the 

 branches is, as a rule, larger than the sectional area of the stem. 

 Thus the collective capacity of the arteries is continually (and 

 rapidly) increasing from the heart towards the capillaries. If all 

 the arterial branches were fused together, they would form a 

 funnel, with its apex at the aorta. The united sectional area of 

 the capillaries has been calculated by Vierordt to amount to 

 several (eight ?) hundred times that of the aorta. 



3. The capillaries, channels of exceedingly small but variable 

 size. Their walls are elastic (as shewn by their behaviour during 

 the passage of blood-corpuscles through them), exceedingly thin 

 and permeable. They are permeable both in the sense of allowing 

 fluids to pass through them by osmosis, and also in the sense of 

 allowing white and red corpuscles to traverse them. The small 

 arteries and veins, which gradually pass into and from the capil- 

 laries properly so called, are similarly permeable, the more so, the 

 smaller they are. 



4. The veins, less elastic than the arteries (the difference beiog 

 especially marked when both sets of vessels become distended) and 

 with a very variable muscular element. The united sectional area of 

 the veins diminishes from the capillaries to the heart, thus resem- 

 bling the arteries ; but the united sectional area of the venae cavae 

 at their junction with the right auricle is greater than that of the 

 aorta at its origin. (The proportion is nearly two to one.) The 

 total capacity of the veins is similarly much greater than that of 

 the arteries. The veins alone can hold the total mass of blood 

 which in life is distributed over both arteries and veins. Indeed 

 nearly the whole blood is capable of being received by what is 

 merely a part of the venous system, viz. the vena portaa and 

 its branches. Such veins as are for various reasons liable to a 

 reflux of blood from the heart towards the capillaries are provided 

 with valves. 



