USES OF THE VALVES OF THE VEINS. 99 



vessel is temporarily obliterated, they aid in directing the current into anas- 

 tomosing vessels. It is but rarely, however, that they act thus in opposition 

 to the force of gravity ; and it is only when many of the veins of a part are 

 simultaneously compressed that they aid in diverting the current. When a 

 single vein is obstructed, it is not probable that the valves are necessary to 

 divert the current into other vessels, for this would take place in obedience to 

 the vis a tergo ; but when many veins are obstructed in a dependent part and 

 the avenues to the heart become insufficient, the valves divide the columns of 

 blood, so that the pressure is equally distributed throughout the extent of the 

 vessels. This is, however, but an occasional action of the valves ; and it is 

 evident that their influence is only to prevent the weight of the entire col- 

 umn of blood, in vessels thus obstructed, from operating on the smallest 

 veins and the capillaries. It can not make the work of the heart, when 

 the blood is again put in motion, any less than if the column were undi- 

 vided, as this organ must have sufficient power to open successively each set 

 of valves. 



It is in connection with the intermittent compression of the veins that the 

 valves have their principal and almost constant use. Their situation alone 

 would lead to this supposition. They are found in greatest numbers through- 

 out the muscular system, having been demonstrated in vessels one line (2-1 

 mm.) in diameter. They are also found in the upper parts of the body, 

 where they certainly do not operate against the force of gravity ; while they 

 do not exist in the cavities, where the venous trunks are not subject to com- 

 pression. It has already been made sufficiently evident that the action of 

 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 regurgitation. 



In the action of muscles, the skin is frequently stretched over the part, 

 and the cutaneous veins are somewhat compressed. 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 are very much less prominent. Here the valves have an important ac- 

 tion. 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 mus- 

 cles into the skin, and the valves act to prevent it from taking a retrograde 

 course. 



A full consideration of the venous anastomoses belongs to descriptive 

 anatomy. It is sufficient to state, in this connection, that they are very 

 abundant and provide for a return of the blood to the heart by a number of 

 channels. The azygos vein, the veins of the spinal canal and veins in the 

 walls of the abdomen and thorax connect the inferior with the superior vena 

 cava. Even the portal vein has been shown to have its communications with 

 the general venous system. Thus, in all parts of the organism, temporary 

 compression of a vein merely diverts the current into some other vessel, and 

 permanent obliteration of a vein produces enlargement of communicating 



