26 WATER AND SOLID CONSTITUENTS OF BLOOD AND SERUM. 



A careful comparison of these results leads to the following conclusions: — 

 1. The proportion of water is greatest in the Invertebrata. The hlood of these 

 animals has, according to Genth, a specific gravity not many degrees above that of 

 common water. 



l 2. Amongst vertebrate animals, the amount of water existing in the blood is 

 greatest in Fishes and Aquatic Reptiles, and least in Serpents, Birds, and Mam- 

 mals. As a necessary consequence, the solid matters of the blood are least in the 

 Invertebrata, Fish, and Aquatic Pteptiles, and greatest in Serpents, Birds, and 

 Mammalia. 



3. It may be laid down as a general law, that as the organs and apparatus of the 

 animal are developed, and the temperature and intellect correspondingly increased, 

 the blood becomes richer in organic constituents. 



The blood of serpents appears, at first sight, to form an exception. The large 

 amount of solid constituents, however, existing in their blood, is readily accounted 

 for, when we consider their habits. These Reptiles seldom or never drink water; 

 consequently, the fluids of their bodies are derived from the animals which they con- 

 sume. In all animals, the water of the blood and tissues is continually evaporating 

 from the surface of the lungs and body. The amount of evaporation is in propor- 

 tion to the structure, habits, and temperature of the animal, and the temperature and 

 moisture of the atmosphere. It is greatest in warm-blooded animals, and in hot 

 and dry climates. Amongst cold-blooded animals, it is greatest in those having 

 naked skins, and least in those covered by scales, bone, and horn. No matter how 

 slow and small this evaporation, if it be not counteracted by a corresponding supply 

 of water, the blood necessarily becomes concentrated, and yields a larger propor- 

 tion of solid constituents upon analysis. 



4. Our knowledge is as yet too limited to develop any laws respecting the 

 amount of water and solid materials which characterize the blood of each species 

 and genus. By comparing the analysis of the blood of the Mammalia, we see that 

 the proportions of its constituents vary as much in individuals of the same species 

 as in individuals of remotely separated genera. 



