302 CIRCULATION. 



situated in proximity to the arteries ; and the other, which 

 is superficial, and receives for the most part the blood from 

 the cutaneous surface. The entire capacity of these vessels, 

 as compared with the arteries, is very great. As a general 

 rule, each vein when fully distended is larger than its adja- 

 cent artery. Many arteries are accompanied by two veins, 

 as the arteries of the extremities; while certain of them, like 

 the brachial or spermatic, have more than two. Added to 

 these is the superficial system of veins which have no corre- 

 sponding arteries. It is true that some arteries have no cor- 

 responding veins, but examples of this kind are not sufficient- 

 ly numerous to diminish, in any marked degree, the great 

 preponderance of the veins, both in number and volume. It 

 is impossible to give an accurate estimate of the extreme ca- 

 pacity of the veins as compared with the arteries ; but from 

 the best information we have, it is several times greater. 

 Borelli estimated that the capacity of the veins was, to the 

 capacity of the arteries, as 4 to 1 ; and Haller, as 2J to 1. 

 The proportion is very variable in different parts of the body. 

 In some situations the capacity of the veins and arteries is 

 about equal ; while in others, as in the pia mater, according 

 to the researches of Hirschfeld, the veins will contain six 

 times as much as the arteries. 1 



In attempting to compare the quantity of blood normally 

 circulating in the veins, with that contained in the arteries, 

 such variations in the venous system at different times and 

 in different parts, both in the quantity of blood, rapidity of 

 circulation, pressure, etc., are found, that a definite estimate 

 is impossible. It would be unphilosophical to attempt an 

 approximate comparison, as the variations in the venous cir- 

 culation constitute one of its greatest and most important 

 physiological peculiarities, which must be fully appreciated 

 in order to form a just idea of the function of the veins. 



1 BERARD, Cours de Physiologic, Paris, 1855, tome iv., p. 7. The circulation 

 in the erectile tissues will be separately considered, and no account is now taken 

 of the relative capacity of veins and arteries in them. 



