THE VEINS. 



129 



A 



them together, there are only two. They are very delicate, 

 consisting of folds of the lining membrane of the vessel, and 

 are quite transparent; but when 

 liquid is injected from the direc- 

 tion of the heart, it is effectually 

 prevented by them from passing 

 back to the twigs. They are 

 nearly absent from the head and 

 neck, and are most abundant in 

 the lower limbs. By prevent- 

 ing regurgitation, they convert 

 the accidental pressure of sur- 

 rounding parts into an auxiliary 

 of the circulation, pushing the 

 blood onwards, but incapable of 

 pushing it backwards. Such 



pressure may, however, easily Fig. 70. VENOUS VALVES. A, 

 be sufficiently great to prevent 

 the entrance of blood into a 

 vein; thus in letting blood from 

 the arm it is customary to make 

 the patient exercise the fingers 

 sufficiently to keep up pressure by contraction of the flexor 

 muscles in the forearm on the deep veins, and so compel the 

 return of the blood by the superficial set. 



It will be perceived from what has been said, that the 

 rate of flow of the blood in individual veins is very variable. 

 Looking, however, at the venous system as a whole, it will 

 be easily understood that it pours into the heart, in a given 

 time, exactly the same amount of blood as is discharged into 

 the arteries; and as the sectional area of the veins is every- 

 where greater than that of the corresponding arteries, the 

 flow of blood within them is in the same proportion slower, 

 probably nowhere more than half as fast; but as, in the 

 arteries, the velocity of the blood is less the nearer it 

 approaches the capillaries, on account of the larger sectional 

 area of the total number of vessels in which it is distributed, 

 so it again increases in the passage from the sir nil to the 

 large veins. 



94. The pressure or force with which the blood is urged on its 



a vein laid open to show the 

 two pouches of a valve. B, 

 an unopened vein, exhibits at 

 a, the dilatation opposite a 

 valve ; and at 5, a closed 

 valve seen from below. 



