154 LECTURE V.. 



according to the chemical peculiarities of the individual 

 juices, certain modifications would arise in these currents. 

 Generally speaking, however, the chemical side of this 

 question has been but little regarded. 



It cannot be denied that there are certain facts which 

 cannot well be explained in any other manner, especially 

 in cases where essential alterations have taken place in 

 the state of concentration of the juices, for example, in 

 that form of cataract which Kunde has artificially pro- 

 duced in frogs by the introduction of salt into their intes- 

 tinal canal or subcutaneous cellular tissue. But in pro- 

 portion as, after a physical study of the phenomena of 

 diffusion, the conviction has been acquired that the mem- 

 brane which separates the fluids is not an indifferent 

 substance, but that its nature exercises a directly con- 

 trolling influence upon the permeating powers of the 

 fluids, it becomes impossible that a like influence should 

 be denied the capillary membrane. We must not, 'how- 

 ever, go so far, as to ascribe to this membrane all the 

 peculiarities observable in the interchange of material, 

 and so explain how it happens, that certain matters 

 which enter into the composition of the blood are not 

 distributed in equal proportion to every part, but leave 

 the vessels at some points in greater, at others in less, 

 quantity, and at others not at all. These peculiarities 

 depend, manifestly, on the one hand, upon the differ- 

 ent degrees of pressure to which the column of blood is 

 subjected in certain parts, and, on the other, upon spe- 

 cial properties of the tissues ; and we are irresistibly com- 

 pelled, both by the consideration of simply pathological, 

 and particularly by that of pharmaco-dynamical, pheno- 

 mena to admit that there are certain affinities existing 

 between definite tissues and definite substances, which 

 must be referred to peculiarities of chemical constitution, 

 in virtue of which certain parts are enabled in a greater 



