FUNCTION OF VEINS. — In function veins may be considered as having a triple office : they are passive organs of circulation ; they are diverticula or reservoirs for blood; and they are agents of absorption.
Circulation in the veins consists of the passive and equable transmission of blood along their tubes. " When we consider the great size of the veins, compared with the arteries, we must conclude that the blood flows but slowly in the venous system ; that, from the narrowness of the trunks of the veins near the heart, the blood must be ac celerated as it approaches the heart ; and that receiving the impulse from the ventricle, it must take a rapid course through the arteries, until, again approaching the extreme branches of the arteries, and passing into the veins, its motion becomes more languid and slow. In youth, as the size of the veins is not in so great a proportion to the arteries as in advanced life, the blood must be in more rapid circulation : but in old age, owing to the largeness of the veins and the accu mulation of blood in them, it moves slowly through the venous system, and is almost stagnant in the dilated veins and in the sinuses.
" There is no pulsation to be observed in the veins, but what they receive from con tiguous arteries. There is no pulsation in the veins because they are removed from the heart ; because they do not receive the shock of the heart's action in their trunk, but only by their widely-spread branches,- because the contraction of the heart and of the arteries so alternate with each other, as to keep up a perpetual and uniform stream of blood into the veins." (Sir C. Bell's Anatomy, vol. h. p. 284.) As diverticula and reservoirs of blood the veins must be considered as performing a most important service in the ceconomy of the cir culation. There are numerous circumstances both in health and disease which either tem porarily or permanently disturb the balance of the circulation, and thereby displace and choke up large quantities of blood ; and, were it not for the reservoirs, diverticula, and lateral compensating channels, which are afforded by large, bulging, and dilatable veins, by the enor mous area of the common venous cavity, and by the numerous and elaborate inosculations and plexuses which these vessels every where exhibit, such interruptions might lead to consequences, permanent, irremediable and destructive.
The whole subcutaneous system of veins must be looked upon as a series of compen satory channels. The cavities of the deep veins are quite sufficient to return the blood that is sent to them by vessels of smaller aggregate calibre. But during muscular ex ertion, &c., these deep vessels are impeded by pressure, and then the blood finds a colla teral route by means of the subcutaneous veins. So again, when from cold, or other causes, the skin-muscle contracts and presses on the cutaneous veins they are more or less emptied, and their office is resumed by the deep veins : they thus keep up a mutual reciprocation of function.
The large internal veins of the body; the dilatations of the veins at the base of the brain ; the large amount of veins in the parenchymatous viscera ; the venous plexuses of the spinal canal, together with the venous cells and tubes in the cancellated tissue of all bones, form a series of diverticula and reser voirs, equal to all emergencies*, where the blood may slowly flow, or recede, or stagnate. But the most striking examples of venous re servoirs are furnished by comparative anatomy. All those animals which have diving habits subject themselves to peculiar disturbance of circulation. The prolonged stoppage of re spiration, but still more the immense pressure of water to which they are exposed, when deeply submerged, mechanically empties all the superficial veins and produces much deep seated and visceral congestion. This is com pensated for by a special arrangement. In diving birds there is great dilatation of the posterior vena cava. In the Grebes and Divers (Podiceps and Colynzbus) the posterior cava is largely dilated in the liver and below, extend ing to opposite the kidneys. In the Cetacea, which are subject to the same influence, enormous dilatable venous plexuses, placed within the abdominal cavity, perform the same office.