CHAP. XXVI.] MECHANISM OF ABSORPTION. 287 



twenty-four inches, and alcohol by thirty-six to forty inches of 

 mercury:" and hence it appears, as this eminent writer re- 

 marks, that " the power of a liquid to filter through an animal 

 membrane bears no relation to the mobility of its particles ; for 

 under a pressure which causes water, brine, or oil, to pass through, 

 the far more mobile alcohol does not pass." 



As pressure promotes the transmission of fluid through a mem- 

 brane in one direction, so it tends to interrupt the passage of the 

 other fluid in the opposite direction, or to apply this to the blood- 

 vessels of the living body, if they are distended by an over great 

 quantity of blood, so that this fluid reacts upon their inner surface, 

 as in the case of plethora, fluids enter them with difficulty from 

 without whereas, if their bulk is diminished by venesection, 

 absorption is comparatively rapid. This conclusion was established 

 by Majendie on good grounds, and it has some illustrations and 

 valuable applications in practice. 



Absorption as influenced by Motion of the Fluid within the Vessels. 

 Fluid may be raised out of a reservoir against gravity, by direct- 

 ing a stream along a membranous canal, which lies immersed in the 

 stagnant fluid. The outer fluid enters the canal by endosmose, and 

 is carried away with a speed proportioned to the velocity of the 

 current. If the fluid in motion is so compressed as to exert much 

 lateral pressure on the wall of the tube, it will rather itself pass 

 outwards, so as to mingle with the fluid at rest, than receive and 

 carry off the latter. If the fluid in motion is also the more dense, 

 or otherwise that towards which the external fluid would flow if 

 both were stagnant, then its motion accelerates the endosmose by 

 constantly bringing on fresh fluid of the original density, so that the 

 first rate of transmission is maintained.* 



It will be scarcely necessary to state in detail the particular 

 bearing which the preceding considerations have on the question of 

 the mechanism of the absorbent process in the living body. It is, 

 however, very evident that they leave us with little more than some 



* In a valuable paper by our friend, Dr. Robinson, of Newcastle (Med. 

 Gazette, 1844) many experiments and arguments are given to shew that 

 absorption goes on rather on the venous side of the capillary network, and in 

 the small veins, than on the arterial side. He considers the motion of the 

 blood to be an influential cause of absorption, by diminishing its pressure 

 outwards on the vascular walls, and thus allowing the external pressure (tnat 

 of the atmosphere and of the surrounding tissues) to predominate. There can 

 hardly be a doubt that the rapidity of absorption would be influenced by the 

 rate of movement of the blood as well as by other mechanical conditions. 



