GENERAL CONCLUSIONS. 



127 



quantities of nutriment, which are so often received into the circulatory system. 

 To accompKsh this efTectually, there must be correspondingly large organs. 



In the Herbivora and Granivora, on the other hand, the intestinal canal is long ; 

 in the case of the sheep, it is more than twenty times the length of the body. In 

 this class of animals, the food requires minute subdivision, and the materials which 

 they contain must go through many metamorphoses before they are ready to supply 

 the wastes and wants of the system. Consequently, their introduction into the 

 circulation is gradual ; the elimination of the products of the disintegrations of the 

 tissues, whose place they supply, must be correspondingly slow, and a much smaller 

 sized organ will perform the same office in them than in the Carnivora. 



Another important cause of this difference between the relative sizes of the 

 kidneys of these classes of animals is to be found in the chemical constitution of 

 the food. 



The food of the Carnivora contains much nitrogen, and in the processes of its 

 compositions and decompositions, for the production and maintenance of animal 

 heat, is not completely consumed, and the resulting compounds, as uric acid, and 

 urea, and ammonia, are eliminated principally by the kidneys. On the other hand, 

 almost all the food of frugivorous and granivorous animals is capable of being 

 ultimately resolved into water and carbonic acid gas, or of being converted into fat. 

 The amount of nitrogenized elements to be eliminated by the kidneys is on this 

 account greater in the former than in the latter. 



The same law applies to the kidneys of cold-blooded animals. It is not, however, 

 so evident at first sight. 



The kidneys of Ophidians and Saurians, carnivorous reptiles, are much larger 

 than those of the herbivorous Gopher [Testudo poJyphemus) ; but the kidneys of 

 carnivorous Chelonians are far smaller than those of the Gopher. How are we to 

 explain this apparent anomaly ? A consideration of the peculiar habits and con- 

 stitution of these animals will answer the question. 



The Gopher lives in a barren and sandy country, and to prevent evaporation 

 from the surface, it is covered with horn at all points where the skin is exposed. 

 The function of the skin is consequently little or nothing, and the kidneys must 

 perform the labor. The carnivorous Chelonians, as the Salt-water Terrapin [Emys 

 Terrapin) , and Yellow-bellied Terrapin {Emys serraia), have a naked skin, and live 

 in the water. The function of the skin in all those reptiles living in the water is 

 far more active than that of those living out of it. That evaporation takes place 

 rapidly through the skin of these Chelonians was conclusively shown by our experi- 

 ments upon the effects of thirst and starvation upon their blood. These animals 

 also do not consume as much food as the Gopher. 



It is evident, from these reasons, that the small size of the kidneys of carnivorous 

 terrapins is connected with their peculiar organization and mode of living ; and 

 they cannot be fairly compared with an animal differing totally from them in its 

 physical and vital endowments, and mode of life. The Gopher should be compared 

 with the carnivorous Ophidians and Saurians, because they all live upon the land, 

 and their tegumentary systems are similar. A reference to the tables shows that 

 the kidneys of the former are far smaller than those of the latter, and the truth of 

 the generalization announced is sustained. 



