290 THE CIRCULATION. 



given signal, the two plugs were suddenly turned in opposite 

 directions, the stream of blood would be turned out of its course 

 (Fig. 95), and made to traverse a long bent tube of glass (d, d, d), 

 before again finding its way back to the lower portion of the artery. 

 In this way the distance passed over by the blood in a given time 

 could be readily measured upon a scale attached to the side of the 

 glass tube. Yolkmann found, as the average result of his obser- 

 vations, that the blood moves in the carotid arteries of warm-blooded 

 quadrupeds with a velocity of 12 inches per second. 



VENOUS CIRCULATION. 



The veins, which collect the blood from the tissues and return it 

 to the heart, are composed, like the arteries, of three coats; an inner, 

 middle, and exterior. In structure, they differ from the arteries in 

 containing a much smaller quantity of muscular and elastic fibres, 

 and a larger proportion of simple condensed areolar tissue. They 

 are consequently more flaccid and compressible than the arteries, 

 and less elastic and contractile. They are furthermore distin- 

 guished, throughout the limbs, neck, and external portions of the 

 head and trunk, by being provided with valves, consisting of fibrous 

 sheets arranged in the form of festoons, and so placed in the cavity 

 of the vein as to allow the blood to pass readily from the periphery 

 toward the heart, while they prevent altogether its reflux in an 

 opposite direction. 



Although the veins are provided with walls which are very much 

 thinner and less elastic than those of the arteries, yet, contrary to 

 what we might expect, their capacity for resistance to pressure is 

 equal, or even superior, to that of the arterial tubes. Milne Ed- 

 wards 1 has collected the results of various experiments, which show 

 that the veins will sometimes resist a pressure which is sufficient to 

 rupture the walls of the arteries. In one instance the jugular vein 

 supported, without breaking, a pressure equal to a column of water 

 148 feet in height ; and in another, the iliac vein of a sheep resisted 

 a pressure of more than four atmospheres. The portal vein was 

 found capable of resisting a pressure of six atmospheres ; and in 

 one case, in which the aorta of a sheep was ruptured by a pressure 

 of 158 pounds, the vena cava of the same animal supported a pres- 

 sure equal to 176 pounds. 



.' Lemons sur la Physiologie, &c., vol. iv. p. 301. 



