The spinal veins are destitute of valves, as are, also, those of the portal system, the he patic veins, those of the heart, kidney, uterus 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 Reptiles 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 any 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 with 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 all retrograde circulation into the smaller vessel. The important object of exclusion of venous blood from the thoracic duct is thus effected.
Valves arc essentially passive organs in the circulation ; and they only avail when mo tion is given to the circulating fluid by other means,—they determine what shall, and :Mat 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 this 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.