THE CIRCULATION. 



297 



the heart-beat will cause the blood-pressure to depend upon the recip- 

 rocal relation between the increase of frequency of the pulse and the 

 volume of the blood ejected. When both vagi are divided or the 

 accelerators are stimulated and the heart-beat is accelerated, it may 

 or may not throw out more blood in a given time. If more blood is 

 expelled by the heart, the diameter of the arterioles remaining the 

 same, the blood-pressure increases. A quicker heart-beat does not 

 always indicate a greater systolic force, and thus does not necessarily 

 mean a greater amount of blood ejected. When the arterial pressure 

 is high the pressure in the left ventricle reflexly stimulates the vagus- 

 center, slows the heart, lessens the output of blood, and thus counteracts 

 the high blood-pressure. Low blood-pressure increases the rate of the 



I I M 



Fig. 97. Variations in Pressure. (LANDOIS.) 



A, Cylindrical tube filled with water, a-b, Outflow tube, along which are 

 placed at intervals the vertical tubes, 1, 2, and 3, to estimate pressure. 



heart-beat. The pressure in the circulatory system varies with these 

 factors as variants. Pressure will be greater with greater heart-force or 

 with greater peripheral resistance. The direction of flow is always 

 from a point of higher to one of lower pressure. 



The further the blood proceeds from that center of circulatory 

 motive power, the heart, the less becomes the pressure exerted by 

 it. It must be greatest, therefore, in the arteries emanating from 

 the heart and least in those veins emptying into the right heart. 

 The decrease is rather gradual along the vascular course until the 

 venae cavas are reached; at their point of entrance into the heart 

 the blood-pressure is frequently found to be negative; that is, below 

 atmospheric pressure. 



Thus, the arterioles will be found to possess a pressure that is 

 peculiar to them, as do the capillaries and veins in their turn. The 

 intensity of the pressure will depend upon the resistances to be over- 



