VEIN. 



1381 



and lungs. Comparative anatomy and acci- 

 dental aberrations from nature furnish some 

 exceptions to the foregoing statements. Meyer 

 has found incomplete valves in the pulmonary 

 veins. Theile has found valves in the ova- 

 rian veins. E. H. Weber has observed them 

 in the portal veins of the horse ; and Cuvier, 

 in the same animal, has found them in the 

 splenic and mesenteric veins. Haller has dis- 

 covered valves in the pulmonary veins of the 

 dog and sheep. 



Valves exist but sparingly in Birds and 

 Cetaceans; and in R> ptiles and Fish are almost 



wanting. 



The office of valves is to prevent the blood 

 from effecting a retrograde course ; " lest, 

 instead of advancing from the extreme to the 

 central parts of the body, the blood should 

 rather proceed along the veins from the 

 centre to the extremities ; but the delicate 

 valves, while they readily open in the right 

 direction, entirely prevent all such contrary 

 motion; being so situated and arranged, that 

 if aiiy thing escapes, or is less perfectly ob- 

 structed by the cornua of the one above, the 

 fluid, passing, as it were, by the chinks be- 

 tween the cornua, is immediately received 

 on the concavity of the one beneath, which is 

 placed transversely witli reference to the 

 former, and so is effectually hindered from 

 getting any farther."* This refers especially 

 to the valves in the venous canals. There is 

 a peculiarity about those placed at the orifices 

 of veins. I have already remarked that all 

 single valves placed at the mouths of veins 

 are attached to the distal margin of the orifice : 

 the free margin looks towards the heart, the 

 concave face obliquely towards the cavity of the 

 tributary, and the convex in the opposite direc- 

 tion. Now it is obvious from this arrange- 

 ment, that when the blood falls back upon 

 the concave surface of such a valve, it throws 

 it more or less across the area of the larger 

 trunk, and away from the orifice of the 

 smaller vessel, over which it forms no pro- 

 tection. The office of the single valve, thus 

 placed, appears to be not to guard the small 

 veins, or to prevent the retrocessant blood 

 from passing into it, but to oppose the blood 

 which it supplies from passing into that por- 

 tion of the recipient vein which is behind the 

 orifice of the tributary : the valve, as it were, 

 directs the blood into the onward track of 

 the great vein, and prevents it from taking an 

 opposite course. 



The case, however, is different where the 

 valvular apparatus is double, for there a valve 

 is placed at the proximal, as well as at the 

 distal margin of the orifice : now the former 

 of these valves prevents the blood from pass- 

 ing behind the other into the smaller vein 

 the two valves mutually assist each other, and 

 prevent the blood from passing behind either; 

 and the result is a complete obstruction to a'l 

 retrograde circulation into the smaller vessel. 

 The important object of exclusion of venous 

 blood from the thoracic duct is thus effected. 



* Harvey, loc. fit. 



Valves are essentially passive organs in the 

 circulation ; and they only avail when mo- 

 tion is given to the circulating fluid by other 

 means, they determine ivhat shall, and what 

 shall not, be the direction of the moving fluid. 

 The contraction of the muscles is one of the 

 great motor agents of the venous circulation. 

 When a muscle contiguous to a vein con- 

 tracts, the vessel is compressed and the blood 

 forced out of it ; and, were it not for the 

 valves placed at the distal side of the com- 

 pressed point, the blood would be sent as 

 much from, as towards, the heart. Autenrieth 

 has put tins in a striking point of view ; 

 he says, " Each swelling muscle becomes 

 thereby, for the neighbouring veins, a kind of 

 heart furnished with valves ; thus, while it 

 presses the vein, the valves prevent the blood 

 in the lower part of the same from being 

 driven backwards ; but, in the upper part, 

 opening valves are placed, from which the 

 blood, driven forth by the contracting muscle, 

 meets with no opposition in its direction to- 

 wards the heart. It is from this circumstance 

 that each violent movement of the body, which 

 consists in an alternate swelling up and sink- 

 ing down of the muscles, has so great a ten- 

 dency to accelerate the circulation ; naturally, 

 however, while the muscles are in a state of 

 repose, the valves cannot facilitate the move- 

 ment of the blood."* 



The function of the sinuses is this, to re- 

 ceive the valves when they are folded back, 

 during the onward current of the blood; and 

 to allow the blood to pass behind them and 

 to throw them across the area of the vessel 

 when that fluid regurgitates. 



Vasa vasorum. The coats of all bloodves- 

 sels, except those of very small size, are sup- 

 plied with arteries and veins especially devoted 

 to their nutrition, called " vasa vasorum." 



According to Henle, vessels (veins and 

 arteries in common) receive vasa vasorum, 

 though themselves not larger than ^ of an 

 inch in diameter, and sometimes even smaller. 

 The coats of capillaries and those of the 

 smallest vessels are not thus nourished by 

 a separate vascular supply. 



The nutrient arteries of the venous coats 

 arc derived from the small arterial trunks in 

 the vicinity of the vessel f ; they do not come 

 directly from larger arterial trunks, but from 

 smaller vessels; and the source of the vascular 

 supply is determined by the particular neigh- 

 bourhood of the vein the same vessel having 

 vasa vasorum from different sources as it passes 

 along different regions : thus, the vena azygos 

 is nourished by the intercostal, pcricardial, and 

 cesophageal, arteries, according to its various 

 relations as it proceeds along its course. The 

 arteries supplying the tunics of a vein, appear 

 to go promiscuously to the vessel and the 

 adjacent tissues, sending some of its ramules 

 to the areolar tissue, nerves, fat, &c., in the 

 neighbourhood, and others to the walls of the 

 vessel. In the accompan)ing figure, repre- 



* Autenrieth. Physiologic, 388, 389. 

 f The artery usually has its vascular supply from 

 the same source as the' vein which accompanies it. 



4 T 3 



