THE CIRCULATION OF THE BLOOD. 331 



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 phy- 

 sical 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 arterial 

 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. 



That the high pressure in the arteries is largely due to this physiologic factor 

 is shown by the rapid and pronounced fall of pressure that occurs when this 

 muscle suddenly relaxes as it does when the spinal cord is transversely 

 divided in the cervical region, thus cutting off from the arteriole muscle 

 those nerve influences that largely determine its contraction. Under 

 such circumstances the pressure in the dog may fall from approximately 

 140 mm. to 40 mm. of mercury or even less. Stimulation of the distal 

 extremity of the spinal cord will be followed by the temporary contraction of 

 the muscle and a rise of pressure to its former value. 



The Distribution of the Intra-ventricular Pressure. The pressure 

 developed during the ventricular contraction is thus expended in imparting 

 velocity to the blood and overcoming the cohesion and friction of its mole- 

 cules. The percentage of the pressure utilized in overcoming the resistance 

 could be approximately determined from the pressure in the aorta if this were 

 accurately known; the percentage of the pressure utilized in imparting 



velocity could be determined with the formula ; if the actual velocity of the 



2 g 



blood in the aorta could be experimentally determined. On account of 

 the difficulty in obtaining this latter factor at least, the results must be only 

 approximative. 



An idea of the ratio between the velocity pressure and the resistance pressure, 

 however, may be obtained from the distribution of the aortic pressure in the 

 dog in reference to the carotid artery. Thus, if it be assumed that the aver- 



age velocity of the blood is 35 cm., the velocity pressure is equal to ~^ or 



0.62 centimeters of blood or 0.046 centimeters of mercury, and if the average 

 aortic pressure is 150 mm. of mercury , the ratio of the velocity pressure to 

 the resistance pressure is as i to 326. 



The phenomena which for the most part characterize the flow of blood 

 through the blood-vessels are velocity and pressure, combined with an 

 alternate expansion and recoil of the arterial vessels due to the intermittent 

 character of the heart-beat. For special reasons it is convenient to consider 

 the pressure first. 



