FUNCTION OF THE VALVES. 327 



muscles seconds most powerfully the contractions of the 

 heart. The vis a tergo from the heart is, doubtless, generally 

 sufficient to turn this influence of muscular compression from 

 the capillary system, and the valves of the veins are open ; 

 but they stand ready, nevertheless, to oppose any tendency 

 to regurgitation. 



In the action of muscles, the skin is frequently stretched 

 over the part, and the cutaneous veins are somewhat conir 

 pressed. This may be seen in the hand, by letting it hang 

 by the side until the veins become somewhat swollen, and 

 then contracting the muscles, when the skin will become 

 tense and the veins very much less prominent. Here the 

 valves have an important action. The compression of the 

 veins is much greater in the substance of and between the 

 muscles than in the skin ; but the blood is forced from the 

 muscles into the skin, and the valves act to prevent it from 

 taking a retrograde course. The fact that the contraction of 

 muscles forces blood into the veins of the skin may be seen 

 by surrounding the upper part of the forearm with a moder- 

 ately tight ligature, which will distend the cutaneous veins 

 below. If we now contract the muscles vigorously, the veins 

 below will become sensibly more distended and knotted; 

 showing, at once, the passage of blood into the skin, and the 

 action of the valves. 



When a vein is distended by the injection of air, or a 

 liquid, forced against the valves, it is observed that at the 

 point where the convex borders of the valves are attached, 

 the vessel is not dilated as much as at other parts. This is 

 due to the fact that the valves are bordered with a fibrous 

 ring, which strengthens the vessel, and prevents distention at 

 that point, which would separate the free borders of the valves 

 and render them insufficient. 



A full consideration of the venous anastomoses belongs to 

 descriptive anatomy. Suffice it to say, in this connection, that 

 they are very numerous, and provide for a return of the blood 

 to the heart by a number of channels. The az} T gos vein, the 



