FUNCTIONS OF ORGANIC OR VEGETATIVE LIFE. 229 



body, is the most injurious. Accordingly, we find two large organs the 

 Lungs and the Liver adapted to remove it ; and to both these Venous blood 

 passes, before it is again sent through the system. The function of the Lungs 

 is so important in warm-blooded animals, that a special heart is provided for 

 propelling the blood through them ; in addition to the one possessed by most 

 of the lower animals, the function of which is the propulsion of the blood 

 through the system. In these organs, the blood is subjected to the influence 

 of the atmosphere, by which the carbonic acid with which it was charged, is 

 removed and replaced by oxygen ; and this change takes place, through the 

 delicate membrane that lines the air-cells of the lungs, according to the physi- 

 cal law of the mutual diffusion of gases. The introduction of oxygen into 

 the blood is necessary for the maintenance of those peculiar vivifying powers, 

 by which the Nervous and Muscular systems are kept in a state fit for activity ; 

 and its union with their elements appears to be a necessary condition of the 

 manifestation of their peculiar powers. Of this union, carbonic acid is one 

 of the chief products ; and we shall find that the demand for oxygen, and the 

 excretion of carbonic acid, vary according to the amount of nervous and mus- 

 cular action. The continual formation of carbonic acid, in this and other in- 

 terstitial changes, appears to have a most important purpose in the vital eco- 

 nomy, that of keeping up its temperature to a fixed standard; for the union 

 of carbon and oxygen in this situation may be compared to a process of slow 

 combustion ; and it is well known that, the more energetic this is, the higher 

 is the temperature. Thus, in Birds, whose muscular and nervous activity is 

 so great, and whose respiration is so energetic, the temperature is constantly 

 maintained at a point higher than that which other animals ever attain, in the 

 healthy state at least ; whilst in Reptiles, which present a condition exactly 

 the reverse of this, the temperature is scarcely above that of the surrounding 

 medium. The function of the Liver is, like that of the lungs, twofold ; it 

 separates from the blood a large quantity of the superfluous hydro-carbon, 

 which it acquires by circulating through the tissues ; and it combines that 

 carbon with other elements, into a secretion, which, as we have seen, is of 

 great importance in the digestive process. The hepatic circulation, however, 

 is not kept up by a distinct impelling organ ; but the venous blood from the 

 abdominal viscera (and, in the lower Vertebrata, that from the posterior part 

 of the body) passes through the Liver on its return to the heart. 



276. All animal substances have a tendency, during their decomposition, 

 to throw off nitrogen, as well as carbon ; and this nitrogen may take the form 

 either of cyanogen, by going off in combination with carbon, or of ammo- 

 nia, by uniting at the time of its liberation with hydrogen. The chief function 

 of the Kidneys is evidently to separate the azotized products of decay from 

 the circulating fluid ; for the secretion which is characteristic of them, namely 

 urea, contains a larger proportion of nitrogen than is found in any other 

 organic compound ; it is identical in its chemical nature with cyanate of am- 

 monia, and maybe considered as the result of the union of these two products 

 of animal decomposition. The action of the kidneys is equally essential to 

 the continued performance of the other vital functions, with that of the lungs 

 and liver; since death invariably follows its suspension, unless some other 

 means be provided by Nature (as occasionally happens), for the separation of 

 its characteristic excretion from the circulating blood. 



277. There seems reason to believe, however, that, of the products of 

 decomposition which are set free in the various tissues and organs of the 

 body, only a part is destined to be immediately excreted ; and that it is this 

 part, which is taken up by the Veins, and conveyed, by the general vascular 

 apparatus, to the several glands which are to separate it. The remainder, 

 consisting of substances which are fit to be re-assimilated, appears to be 



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