270 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



the force and frequency of the heart's beat causes a rise of arterial 

 pressure. 



Again, if more fluid is poured into E, there is a correspond- 

 ing 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 means of the clip 

 E ; if the clip is tightened, one imitates increased constriction of the 

 arterioles ; if it is loosened, one imitates dilatation of the arterioles. 

 If it is closed entirely, the fluid in A and B rises to the same level as 

 that in E ; the pressure of E is not felt at all by C and D, 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. 237. 



AUo' 



FIG. 237. Schema of the circulation. 



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

 worked with the hand ; the tube from it represents the arterial system, 

 the clip E the resistance of the arterioles ; C is the capillary lake, 

 from which the vein (larger than the artery) leads back to the heart 

 H. A and B are two manometers which respectively indicate arterial 

 and venous pressures. Only in place of straight tubes mercurial 

 manometers are used. Each of these is a |J~tube about half filled 

 with mercury, and united to the artery or vein by a tube containing 

 fluid. If the mercury in the two limbs of the U is at the same level, 

 the pressure of the fluid in connection with one limb is exactly equal to 



