ABSORPTION. 255 



stream, we can scarcely observe even a faint red tint in the solution 

 of sulphocyanide of potassium. 



If we observe far more favourable conditions in the sources of 

 absorption in the intestine than in the roots of plants, this is 

 mainly owing to the circumstances by whose reaction the con- 

 tinuance of the absorption is controlled. It need hardly be 

 observed that the rapid absorption of aqueous fluid would very 

 soon thin the blood and the whole mass of the juices to such an 

 extent as finally to put a stop to any further endosmotic action. 

 But the animal, and more especially the human body, presents two 

 means for removing the excess of water from the blood ; one of 

 these is tolerably analogous with what occurs in vegetables, whilst 

 the other is peculiar to certain of the higher animals, including 

 man. Like the leaves of plants, the lungs, and in part also the 

 skin of animals, present so large an evaporating surface to the 

 atmosphere, that here, as in the leaves, an extraordinary quantity 

 of water is volatilised. According to Lindenau's approximate 

 calculation, the surface of the lungs of an adult man amounts to 

 2,642 square feet, whilst the surface of the skin cannot be 

 estimated at more than 12 square feet, and even if we assume the 

 area of the much-plaited internal surface of the intestine to 

 measure 24 square feet, the excessive difference between the 

 absorbing and the evaporating surface will be sufficiently mani- 

 fested to elucidate this admirably contrived mechanism. The 

 evaporating surface is not, however, so readily exposed to the air 

 in animals as in plants ; for, even under ordinary circumstances, 

 large portions of the evaporating membranes are so closely 

 approximated and even collapsed together, that they are rendered 

 almost inefficient ; for the pure, comparatively dry atmosphere does 

 not come in direct contact with these evaporating surfaces, but in 

 general only a mixture of air considerably impregnated with 

 aqueous vapour. To this we must add, that the ingestion of fluid 

 food is, to a certain extent, a voluntary act in animals, and hence a 

 much larger mass of fluid may readily be conveyed to the intestine 

 than the pulmonary and cutaneous evaporation can remove, a cir- 

 cumstance which might readily induce a disturbance in the whole 

 mechanism. Finally, we must remember that a large quantity of 

 water is generated in the animal even by its vital processes, which 

 must contribute towards the attenuation of the juices, whilst 

 organic substances are generated in the plant by the decomposition 

 of water, and the juices of the cells are thus rendered more highly 

 concentrated. 



