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CHAPTER XVI 



SCHEMA OF THE CIRCULATION. THE SPHYGMOGRAPH 



The flow of the blood through the arteries, capillaries, and veins 

 follows the laws which govern the flow of any fluid through a system 

 of tubes, so that we are able to illustrate many of the features of 

 the blood flow upon an artificial arrangement of tubes drawn up in 

 imitation of the circulation. Such an artificial system is known as a 

 Schema of the circulation. On examining the conditions of flow 

 in the blood vessels, we find that if we expose an artery it feels hard 

 and distended to the touch, and that synchronously with each heart 

 beat it swells and becomes harder. If we cut into an artery the 

 blood spurts out with considerable force which carries it to some 

 distance, and in addition the outflow is of variable rate, for with each 

 beat of the heart the flow is markedly accelerated. To stop the flow 

 it is necessary either to tie or compress the vessel at some point 

 nearer to the heart than the orifice which has been made into it. If, 

 on the other hand, we lay bare a vein, it is found to be collapsed and 

 very readily compressed, and when its cavity is completely oblite- 

 rated by compression it swells up on the side farthest from the heart. 

 If we cut into a vein the blood flows from it at a good rate, but with 

 slight force and in a constant stream, and to stop this flow it is 

 necessary to tie the vein at any point of the vessel further from the 

 heart than the incision. There are thus many great differences in 

 the characters of the flow from an artery and vein respectively. This 

 change in the nature of the flow is brought about either at the com- 

 mencement of the capillaries or in the small terminal arteries, for if 

 we examine the capillaries in a living animal under a microscope, the 

 flow is found to be constant and not alternately fast and slow. As 

 arteries subdivide, though each branch may be much smaller than the 

 main trunk, yet the total transverse sectional area is invariably found 

 to be greater than that of the main artery. Again, when we pass 

 from the small arteries to the capillaries, though each capillary is of 

 very small sectional area, their number is so immense that their total 

 sectional area is many hundred times greater than that of the arteries 



