n VASOMOTOR NERVES 65 



driven by the same force, viz. the heart's beat, over the 

 whole body, must pass much more rapidly through the 

 aorta than through tlie capillary system or any part of 

 that system. 



It is not that the greater friction in any capillary 

 causes the blood to flow more slowly there and there 

 only. The resistance caused by the friction in the 

 capillaries is thrown back upon the aorta, which indeed 

 feels the resistance of the whole vascular system ; and it 

 is this total resistance which has to be overcome by the 

 heart before the blood can move on at all. 



The blood driven everywhere by the same force simply 

 moves more and more slowly as it passes into wider and 

 wider channels. When it is in the capillaries it is slowest ; 

 after escaping from the capillaries, as the veins unite into 

 larger and larger trunks, and hence as the^ total venous 

 area is getting less and less, the blood moves again faster 

 and faster for just the same reason that in the arteries it 

 moved slower and slower. It is, in fact, the differences in 

 the undth of the ''bed," and this alone, which determines 

 the differences in the rate of flow at the various points of 

 the vascular system. 



A very similar case is that of a river widening out in 

 a plain into a lake and then contracting into a narrow stream 

 again. The water is driven by one force throughout (that 

 of gravity). The current is much slower in the lake than 

 in the narrower river either before or behind. 



14. The Nervous Control of the Arteries. Vaso- 

 motor Nerves. — The arteries, as we have seen, are char- 

 acterised structurally by being elastic and muscular. In 

 the large arteries the elastic properties are more marked 

 than the muscular, whereas in the smaller arteries the 

 muscular tissue is present in large amount relatively to 

 the elastic elements ; and we have dealt in detail with the 

 significance of arterial elasticity and its use in connection 

 with the establishment of blood pressure and the dis- 

 appearance of the pulse. It has also been pointed out 



