THE 1N^'ER TISSUES OF ANIMALS. 327 



vitb. the air, must be in some degree modified by the 

 Bction of the air ; and the directly-produced modification, 

 increasing in the individual and in successive individuals, 

 cannot cease until there is a complete balance between the 

 actions of the changed agency an^ the changed tissue. It i? 

 indeed probable that the growth as well as the differentiatioc 

 of the pulmonic surface, when once commenced, will be 

 furthered by the direct process. The reasoning befon 

 used in the case of branchiae (§ 292) applies in the casi 

 of lungs. If exchange between the plasma in the blood- 

 vessels and the plasma in the tissues surrounding them_ 

 goes on with a rapidity that becomes greater where the 

 difference between them becomes greater ; if, consequently, 

 at some place where the carbonized plasma inside the 

 blood-vessels is brought close to an unusually decarbonized 

 or much oxygenated plasma outside of the blood-vessels, the 

 exchange of these liquids becomes unusually active ; if, as a 

 result, the circulation in the part is augmented ; then it is to 

 be inferred that the extra nutrition will cause extra growth. 

 The surface of the rudimentar}' lung will increase in area so 

 long as the capillary osmose is much greater than in other 

 parts of the body ; and it will continue to be greater until, 

 by the extension of the aerating surface, the respiratory 

 exchange has been rendered so efiicient as to bring down the 

 contrast between the intra- vascular and extra- vascular liquids 

 to a level with the contrasts between the intra-vascular and 

 extra-vascular liquids in other organs. That is to say, the 

 growth which this direct action produces, will go on until the 

 functional efhcienc}^ of the lungs is in equilibrium with the 

 functional efficiencies of other parts throughout the organism. 



§ 300. We come now to differentiations among the truly 

 inner tissues — the tissues which have direct converse neither 

 with the environment nor with the foreign substances taken 

 into the organism from the environment. These, speaking 

 broadly, are the tissues which lie between the double layer 



