256 DIGESTION. 



Owing to these relations, it may readily and frequently happen 

 that the evaporation is insufficient to counteract the absorption in 

 the higher animals ; and to meet this condition, we find in these 

 animals a mechanism which we are at present unable to explain, 

 but the purpose of which is to remove, in a fluid condition, the 

 excess of water from the blood. We need scarcely refer here to 

 the part taken by the kidneys in accomplishing this function in 

 the higher animals, although this is absent in birds, which drink 

 only little fluid, whilst they exhale a large quantity of aqueous 

 vapour during rapid evaporation, and in the lower animals, which 

 do not drink, and exist under peculiar relations. 



This brief notice of the mechanical relations existing between 

 evaporation and the ingestion of fluids into the animal body, is 

 sufficient to show that by the clearer exposition of several physical 

 laws, with which we are still but imperfectly acquainted, we have 

 made an important advance towards the knowledge of the mecha- 

 nical effects exhibited in the animal body. The process of ab- 

 sorption appears to be so wonderfully simple in all its details, that 

 we can scarcely comprehend at the first glance why nature has 

 thus superfluously added to the capillaries other and special ab- 

 sorbing vessels, namely, the lacteals. Whilst in a past age the 

 transition of fluids from the intestine to the kidneys was con- 

 jectured to take place through " vice clandestine" we may now 

 examine the "vice apertae" through which the liquefied nutrient 

 matter passes into the blood, and which, although not entirely 

 devoid of purpose, appear to us almost superfluous when we re- 

 member the power of absorption possessed by the blood-vessels. 

 Our knowledge of this fact should teach us not to overlook those 

 phenomena which we cannot freely deduce from the known pro- 

 positions of the statics and dynamics of molecular motions, and 

 should remind us that cases very frequently occur in physiological 

 as well as pathological conditions, where the capillaries will appear 

 to be either unsuited or inadequate for the purpose of absorption, 

 when judged by the endosmotic actions with which we have already 

 become acquainted. It too often happens, that when a beautiful 

 physical discovery has been made, it has been hastily applied to 

 all analogous relations in the living organism, without considering 

 that the sum of the existing conditions must give rise to the most 

 various modifications of the newly discovered physical proposition. 

 Thus, for instance, on considering more carefully the process of 

 resorption through the intestinal veins, we are struck by a suc- 

 cession of contradictions, which do not admit of being referred 



