THE CIRCULATION OF THE BLOOD 341 



the larger blood-vessels the increase .is inversely proportional to the simple 

 diameter; the velocity will again increase from the middle of the capillary 

 system to the ends of the venae cavae in a ratio again proportional to the 

 total area of each cross-section of the stream -bed until in the vense cavae it 

 will attain its maximal value, though it will not attain its initial value in these 

 vessels because their combined sectional area is greater than that of the aorta. 



The Lateral Pressure. The lateral pressure will also gradually fall 

 from the beginning of the aorta to the ends of the venae cavae, though the 

 fall will be most rapid at the periphery of the arteries. In the arterial system 

 the fall of pressure will be proportional to the ratio between the increase in 

 resistance due to the narrowing of individual vessels, and the decrease in 

 resistance due to the widening of the stream-bed; as the former preponder- 

 ates over the latter there must be an increase in resistance from the aorta 

 to the capillaries and hence a sharper fall of pressure toward the termination 

 of the arterioles, which is very steep for reasons to be stated later. In the 

 venous system the fall of pressure will continue and its rate will be propor- 

 tional to the ratio between the increase in resistance due to the narrowing of 

 the stream-bed, and the decrease of resistance due to the enlarging of indi- 

 vidual vessels; as the latter preponderates over the former there should be 

 a rapid fall of pressure from the capillary system to the ends of the venae 

 cavae. This, however, is to some extent prevented for the reason that the 

 increase in velocity due to the narrowing of the stream-bed increases the 

 resistance to a relatively high value and hence the pressure falls less rapidly 

 than it otherwise would. 



The Resistance. The resistance to the flow of blood through the sys- 

 temic vascular apparatus is due to the cohesion and friction of its molecules 

 as well as that of the corpuscles and the adhesion of the blood to the sides of 

 the vessels. The resistance is also increased by the small diameter of the 

 capillary blood-vessels. 



The high* pressure characteristic of the arterial system contrasted 

 with the low pressure characteristic of the venous system determined by 

 experiment, cannot be accounted for alone by the resistance offered by the 

 small diameter of the vessels of the capillary system. This in itself would be 

 insufficient to maintain the observed differences in pressure in the different 

 sections of the vascular apparatus necessary for physiologic purposes. To 

 meet this necessity there has been developed at the periphery of the arterial 

 system, in the arteriole wall, a special muscle, a, x, Fig. 152 which by con- 

 tracting can add a physiologic resistance to what might be termed the phys- 

 ical resistance of the system. According to the degree of its contraction 

 will the resistance to the flow of blood from the arteries to the veins at the 

 periphery of the arterial system be increased and the arterial pressure be 

 raised and the venous pressure be lowered. According to the degree of its 

 relaxation will the resistance to the flow of blood from the arteries into the 

 veins be decreased at the periphery of the arterial system and the arteria) 

 pressure be lowered and the venous pressure raised. By this means the 

 extent and the relation of the pressure in the two main sections of the 

 systemic vascular apparatus can be temporarily or permanently changed in 

 one direction or the other. The effect of the diminution in the caliber of 

 the arteriole due to the contraction of the muscle is spoken of as the 

 peripheral resistance. 



