CIRCULATION OF BLOOD IN THE VEINS 



CIRCULATION OF BLOOD IN THE VEINS 



The blood, distributed to the capillaries of all the tissues and organs 

 by the arteries, is collected from these parts in the veins and carried 

 to the right side of the heart. In studying the anatomy of the capil- 

 laries or in observing the passage of the blood from the capillaries to 

 larger vessels in parts of the living organism that can be submitted to 

 microscopical examination, it is seen that the capillaries vessels of 

 nearly uniform diameter and anastomosing in every direction empty 

 into a system of vessels, which, 

 by union with others, become 

 larger and larger, and carry the 

 blood away in a continuous cur- 

 rent, in which the intermittent 

 impulse derived from the left 

 ventricle is not manifest. These 

 vessels are called venules, or 

 venous radicles. They are the 

 peripheral radicles of the vessels 

 that cafry the blood to the heart. 



The venous system may be 

 considered as divided into two 

 sets of vessels : one, which is 

 deep-seated and is situated in . .. 



Fig. 26. Venous radicles uniting to form a small 

 proximity tO the arteries, and vein, from the muscular coat of the urinary bladder of 



' 



the other, which is superficial 



and receives the greatest part This preparation shows the endothelium of the 

 Of the blood from the CUtaneOUS vessels - Jt is injected with silver nitrate and mounted 



.. in Canada balsam. 



surface. The entire capacity of 



the venous system, as compared with that of the arteries, is very great. 

 As a 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, are the superficial veins, which 

 have no corresponding arteries. It is true that some arteries have no 

 corresponding veins, but examples of this kind are not sufficient in 

 number to diminish, in any marked degree, the great preponderance of 

 the veins, both in number and volume. It is impossible to give any- 

 thing like an accurate estimate of the extreme capacity of the veins as 

 compared with the arteries, but it must be much greater. Borelli 

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

 arteries, as 4 to i ; and Haller, as 2\ to I. The proportion is variable 



