CH. XXI.] 



BLOOD-PRESSURE. 



283 



tube is collected in a jug and poured back into R we complete 

 the circulation. But the schema is an extremely rough one, and 

 is especially faulty in that the pressure which starts at R is nearly 

 constant and not intermittent. This may be remedied by taking 

 R in the hand, and raising and lowering it alternately. The fluid 

 in the manometers bobs up and down with every rise and fall of 

 R : this is least marked in D. The greater and the faster the 

 movement of R, the greater is the rise of arterial pressure. This 

 is a rough illustration of the fact that increase in the force and 

 frequency of the heart's beat causes a rise of arterial pressure. 

 Again, if more fluid is poured into R, there is a corresponding 



Fig. 272. Schema of the circulation. 



rise in fluid in the manometers. This illustrates the rise of pressure 

 produced by an increase in the contents of the vascular system. 



And this schema, rough though it is, also serves to illustrate 

 the third important factor in the maintenance of the blood- 

 pressure, namely, the peripheral resistance. This is done by 

 mr.ms of the clip E ; if the clip is tightened, one imitates increased 

 constriction of the arterioles ; if it is loosened, one imitates dilata- 

 tion of the arterioles. If it is closed entirely, the fluid in A and 

 H rises to the same level as that in R; the pressure of R is not 

 felt at all by C and I), which empty themselves, and the flow 

 ceases. If the clip E is only tightened so as not to be quite 

 closed, the arterial pressure (in A and B) rises, and the venous 

 pressure (in D) falls ; if the clip is freely opened, the arterial 

 pressure falls, and the venous pressure rises. 



These same facts can be demonstrated by a more perfect circula- 

 tion schema such as is represented in fig. 272. 



The heart (H) is represented by a Higginson's syringe, which is 



