34 



VELOCITY OF THE BLOOD. 



tied into the central, and p into the peripheral, part 



tjnei 



of a divided artery. As the blood flows 

 through the tube, the blood rises higher in a than b. 



To avoid having the manometers a and b too long, they are connected with each other by a 

 capillary tube filled with air and provided above with a stop-cock i. The blood is allowed to 

 j- rise to the height of 1 and 2, the stop-cock t 



* is closed, and practically an air-manometer is 



made, which snows a marked difference in the 

 level of the blood of the two tubes. The level 

 of the blood in 1 and 2 is continually changed 

 by the movements of the heart and those of 

 respiration, and these variations are photo- 

 graphed by means of a camera n with a rapidly 

 moving plate k. 



Fig. C shows a curve obtained from 

 the carotid of a dog. The velocity of 

 the current at 1 2 1 = 238 mm., in the 

 phase 2j-2 = 225 mm., and at 3j-3 = 177 

 mm. The velocity is greatest at the end 

 of inspiration and the beginning of ex- 

 piration. Asphyxia increases it at first. 

 Paralysis of the sympathetic increases 

 it, while stimulation of this nerve 

 diminishes it. Section of the vagi in- 

 creases the velocity, while stimulation 

 diminishes it. 



The curve of the velocity may be written 

 off on a smoked glass plate, moving parallel 

 with the index b. The dromograph curve, III, 

 shows the primary elevation, P, and the di- 

 crotic elevation R. 



90. VELOCITY OF THE BLOOD. 



(1) Division of Vessels Arteries. 



In estimating the velocity of the blood, 

 it is important to remember that the 

 sectional area of all the branches of the 

 aorta becomes greater as we proceed 

 from the aorta towards the capillaries, 

 so that the capillary area is 700 times 

 the aorta. As the veins join and form larger 

 trunks, the venous area gradually becomes smaller, but the sectional area of the 



venous orifices at the heart is greater than that 

 of the corresponding arterial orifices. [We may 

 represent the result as two cones placed base 

 to base (fig. 115), the bases meeting in the 

 capillary area. The sectional area of the ven- 

 ous orifice (V) is represented larger than that 

 of the arterial (A). The increased sectional 

 area influences the velocity of the blood- 

 current, while the resistance affects the pres- 

 sure.! 



Fig. 114. 



I.' Scheme of the photohjematachometei 



II. Pitot's tube. 



greater than the sectional area of 



Fig. 115. 



Scheme of the sectional area. A, arterial 



and V, venous orifice. 



The common iliacs are an exception ; the sum of 



their sectional areas is less than that of the aorta ; 



the sections of the four pulmonary veins are together 



less than that of the pulmonary artery. 



(2) Sectional Area. An equal qucmtity of blood must pass through every section 



of the circulatory system, through the pulmonic as well as through the systemic 



circulation, so that the same amount of blood must pass through the pulmonary 



