88 



CIRCULATION OF THE BLOOD IN THE VESSELS. 



that the capacity of the veins was to the capacity of the arteries, as 4 to 1 ; 

 and Haller, as 2% to 1. The proportion is very variable in different parts of 



FIG. 37. Venous radicles uniting to forma small rein, from the muscular coat of the urinary bladder 

 of the frog ; magnified 400 diameters (from a photograph taken at the United States Army Medical 

 Museum). 



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

 carmine and mounted in Canada balsam. 



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. 



In attempting to compare the quantity of blood normally circulating in 

 the veins with that contained in the arteries, such variations are found at 

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

 of circulation, pressure etc., that a definite estimate is impossible. It would be 

 unprofitable to attempt even an approximate comparison, as the variations in 

 the venous circulation constitute one of its most important physiological 

 peculiarities, which must be fully appreciated in order to form a just idea of 

 the uses of the veins. The arteries are always full, and their tension is sub- 

 ject to comparatively slight variations. Following the blood into the capil- 

 laries, important modifications in the circulation are observed, with varying 

 physiological conditions of the parts. As would naturally be expected, the 

 condition of the veins varies with the changes in the capillaries from which 

 the blood is received. In addition to this, there are independent variations, 



