CIRCULATION OF THE BLOOD IN THE VEINS. 93 



origin, so to speak, to a system of vessels, which, by union with others as we follow their 

 course, become larger and larger, and carry the blood away in a uniform current. These 

 are called the venules, or venous radicles. They are the peripheral radicles of the 

 numerous vessels which transport the blood,' after it has served the purposes of nutrition 

 or secretion, to the central organ. 



The venous system may be considered, in general terms, as divided into two sets of 

 vessels; one, which is deep-seated and situated in proximity to the arteries, and the 



FIG. 33. Venous radicles, uniting to form a small vein, from the muscular coat o/tJie urinary bladder of the 



frog; magnified 400 diameters. (From a photograph taken at the United States Army Medical Museum.) 

 This preparation shows the epithelium of the vessels. It is injected with nitrate of silver, stained with carmine, 



and mounted in Canada balsam. 



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 that of the arteries, is 

 very great. As a general rule, each vein, when fully distended, is larger than its adjacent 

 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 corresponding arteries. It is true 

 that some arteries have no corresponding veins, but examples of this kind are not suffi- 

 ciently 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 

 capacity 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, the veins will contain, when 

 fully distended, six times as much as the arteries. 



