362 PHYSIOLOGICAL DEVELOPMiiNT. 



rationale of these phenomena. When treating of the de- 

 velopment of respirator)^ surfaces, external or internal, at 

 places where the greatest contrast exists between the oxy- 

 genated plasma outside the vessels and the carbonized blood 

 inside them, reference was made to the truth that the ex- 

 change of liquids must, other things equal, be rapid in pro- 

 portion as the c(mtrast between them is great. Now this 

 truth holds generally. In every tissue the rate of osmotic 

 exchange must vary as this contrast varies ; and where the 

 contrast is produced by composition or decomposition going 

 forward in the tissue, the amount of exchange must be pro- 

 portionate to the amount of composition or decomposition. 

 If the blood is circulating through an inactive organ, there 

 is nothing to disturb, in any gre it degree, the proximate 

 equilibrium between the plasma within the blood-vessels and 

 the plasma without them. But if the tissue is functionally 

 excited — if it is made to yield up and expend part of the force 

 latent in its molecules or the molecules of the 0x3^ -hydro- 

 carbons permeating it, its contained liquid necessarily becomes 

 charged with molecules of another order — simpler molecules ; 

 and the greater the amount of function the more different 

 is it made from the liquid contained in the blood-vessels. 

 Hence the osmotic exchange mu-^t be most rapid where the 

 metamorphosis of substance is most rapid — the materials for 

 consumption and for re-integration of tissue, must be supplied 

 in proportion to the demand. This, however, is not the sole 

 process by which waste and repair are equilibrated. Tliere 

 is the osmotic distension above pointed out as one of the 

 causes of circulation — a force tending ever to thrust most 

 blood to the places where there is the greatest escape for it ; 

 that is — the greatest consumption of it. For since in an active 

 tissue, the plasma passing out of its capillaries into its sub- 

 stance is continually yielding up its complex molecules, 

 either to be assimilated or to be decomposed ; and since the 

 products of decomposition, whether of the nitrogenous tissue 

 or of its contained hydro-carbons, are simpler than tba 



