VENOUS ABSORPTION. 617 



ining the blood in the veins, when that salt has been detected in it. 

 Absorption also occurs through the portal veins. The pulmonary veins 

 likewise absorb; for prussiate of potash, in solution, introduced into 

 the trachea, appears sooner in the left cavities of the heart, to which 

 the blood returns from the lungs, than in the right cavities, to which 

 the blood returns from the body generally. Absorption by the pul- 

 monary vessels also takes place in the passage of dissolved oxygen into 

 the blood during respiration. 



In absorption by the bloodvessels, the dissolved substance passes 

 through the thin walls of the capillaries, or finest venules, and so enters 

 the circulation; but as these vessels are always covered by tissue, 

 sometimes exceedingly thin, as in the air-cells, and sometimes thicker, 

 as in the cutis, the absorbed substances not only pass through the 

 coats of the vessels, but must also permeate this overlying tissue. This 

 part of the absorptive process, corresponds with that form of absorp- 

 tion which occurs in the non-vascular tissues and in animals destitute 

 of vessels. Absorption never takes place through the open mouths of 

 vessels, as was formerly supposed; but, instead, a process of permeation 

 occurs through the living tissues, physically identical with that of 

 dialysis through dead animal and other moist permeable membranes, 

 out of the body. This permeation is determined generally, first, by 

 the tendency of different solutions to mix together, or of certain sub- 

 stances contained in the fluid on one side of the membrane, to pass 

 into the fluid on the other side, which does not contain them ; and, 

 secondly, by certain chemical relations between the membrane and the 

 substances applied to it, so that the membrane will permit some things 

 to pass through it more readily than others. The rapid dialysis of 

 acids, salt, sugar, and other substances, as proved by their quick pro- 

 duction of flavor in the mouth, and the equally rapid passage of saline 

 and metallic poisons, especially of the vegetable alkaloids, cyanide of 

 potassium, prussic acid, and many other foreign and noxious substances, 

 into the blood, corresponds with their crystalloid character; whilst the 

 inert colloidal gum, and albumen, are slowly absorbed and are almost 

 tasteless. The removal of the dialyzed material, from beyond the 

 septum, increases the rapidity of the process; and thus also, the nat- 

 ural process of absorption is more rapid, the quicker the circulation in 

 a part ; for the constant renewal of the blood keeps up the required 

 difference between that fluid and the solution of the foreign material, 

 arid the quicker the circulation the more rapid and complete is the re- 

 newal of the blood. 



Absorption by the bloodvessels is necessarily favored by the thin- 

 ness of the layer of tissue which covers them, and is opposed by a 

 thicker and denser covering ; thus, absorption is very rapid from the 

 lungs and peritoneum, quick also from the gastric and intestinal mucous 

 membrane, not quite so quick from the exposed surface of the cutis, 

 whilst it is almost entirely arrested when this is covered by the cuticle. 

 It is very rapid from the subcutaneous cellular tissue, where solutions 

 injected artificially come into almost immediate contact with the walls 

 of the capillaries and venules. Absorption takes place, though slowly, 

 through the coats of even the larger veins, as has been shown, by ex- 



