THE CIRCULATION OF THE BLOOD 



367 



found the velocity 225 mm. per second, which was about one-half that in 

 the aorta of the same animal. The reason for the slow rate of movement in 

 the jugular vein is to be found in the fact that the sectional area of the com- 

 bined venae cavae is about twice that of the aorta; hence the relation of the 

 sectional area of the capillary system to the sectional area of the venae cavae 

 is about 300 to i. 



The blood-pressure, the velocity of the blood, the sectional area of the 

 vascular apparatus, and their relation one to the other are shown in Fig. 168. 



The Relations of Blood-pressure and Velocity. Though the pres- 

 sure of the blood bears a definite relation to the velocity it must be kept in 

 mind that it is rather the difference in pressure between the beginning and 

 the termination of the arterial system, rather than the mean pressure that 

 influences the velocity. Thus, with a given force of the heart and a given 



Arteries. Capillaries. 



FIG. 1 68. , Blood-pressure. , Velocity. 



Veins. 



-o o, Sectional area. 



peripheral resistance, the velocity will have a given value, and so long as 

 these factors remain constant will the velocity remain constant, even though 

 the mean pressure should fall, as from a hemorrhage, or should rise, as from 

 an injection of some indifferent fluid. 



If, however, the primary factors, viz. : the cardiac force or the peripheral 

 resistance, change their values in either the same or opposite directions, 

 there will be a change at once in the velocity. The variations in pressure 

 and velocity, both in the same and opposite directions, which are theoretically 

 possible from a change in the force of the heart, or in the peripheral resistance 

 or both, are shown in the following table arranged by Waller. The plus 

 sign indicates increase, the minus sign, decrease, in effect. 



The statements herein embodied have been established by Marey with an 

 artificial schema of the circulatory apparatus, and by Chauveau and Lortet 

 by experiments on animals with the hemodromograph, a specially devised 

 apparatus for this purpose. 



Though all the relations between pressure and velocity in the table are 

 possible, those which are most physiological are probably 5 and 6, for in both 

 instances there is a minimum alteration in pressure, but a maximum altera- 

 tion in blood flow or velocity. The first instance is the condition most 

 favorable for the functional activity of organs, for the reason that the volume 

 of blood which the organ receives in a unit of time is increased without any 



