FUNCTIONS OF VEGETATIVE LIFE. 81 



an impossibility, when the fact is made known, that no formation of nervous 

 matter takes place in the embryonic structure, until the processes of organic 

 life have been for some time in active operation. The influence which the 

 Nervous System is known to have upon the Function of Nutrition, is probably 

 exerted, rather through the medium of its power of regulating the diameter of 

 the arteries and capillaries, by which it controls in some degree the afflux of 

 blood, and of affecting those preliminary actions on which the quantity and 

 quality of the nutritious fluid depend, than in any more direct manner. At 

 any rate, it may be safely asserted, that no such proof of its more direct influ- 

 ence, as is required to counterbalance the manifest improbability which has 

 been shown to attend it, has yet been given, all the facts which have been 

 adduced in support of this hypothesis being equally explicable on the other, 

 which, being in itself more probable, ought to be preferred. 



92. The renewal which the various tissues of the body are continually 

 undergoing, has for its chief object the counteraction of the decay, into which 

 they would otherwise speedily pass ; and it is obviously required that a means 

 should be provided for conveying away the waste, as well as for supplying 

 the new material. This is partly effected by the venous circulation, which 

 takes up a large part of the products of incipient decomposition, and conveys 

 them to organs where they may be separated and cast forth from the body. 

 The first product of the decay of all organized structures, is carbonic acid ; 

 and this is the one which is most constantly and rapidly accumulating in the 

 system, and the retention of which, therefore, within the body, is the most in- 

 jurious. 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 physical law of the mutual 

 diffusion of gases. The introduction of oxygen into the blood seems necessary 

 alike to maintain its general vivifying powers, and to remove the carbon set 

 free in the tissues, by converting it into carbonic acid; which corresponds 

 with the general fact, that carbonic *acid cannot be formed by decomposition, 

 at least to any large amount, except when the decaying substance has oxygen 

 within its reach. The continual formation of carbonic acid in the tissues, ap- 

 pears to have a most important purpose in the vital economy, 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 nutrition is so active, 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, two-fold : it separates from the blood a large quantity of the 

 superfluous 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 



