FUNCTIONS OF VEGETATIVE LIFE. 367 



son why such a variety of products should spring up in the first instance ; when 

 the cells in which they all originate appear to be so exactly alike. The im- 

 portant discoveries now referred to are not confined to healthy structures; for 

 it has been ascertained that diseased growths have a similar origin and mode of 

 extension, and that the malignant character assigned to Cancer, Fungus Hae- 

 matodes, and other such productions, is partly connected with the fact that 

 they are composed of cells which undergo little metamorphosis, and retain their 

 reproductive power; so that from a single cell, as from that of a Vegetable Fun- 

 gus, a large structure may rapidly spring up, the removal of which is by no 

 means attended with any certainty that it will not speedily reappear, from some 

 germs left in the system. The independent vitality of the cells in which all 

 organized tissues originate, might be of itself a satisfactory proof that, in Ani- 

 mals, as in Plants, the actions of Nutrition are effected by the powers with 

 which they are individually endowed; and that, whatever influence the Nervous 

 system may have upon them, its agency is not essential to their performance. 

 Moreover, it is certain 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 in two ways; first, through 

 its power of regulating the diameter of the arteries and capillaries, by which it 

 controls in some degree the afflux of blood; and secondly, through the more 

 direct relation of the Nervous force to those other forms of Vital agency, which 

 manifest themselves in the growth, development, and maintenance of the living 

 tissues ( 352). 



382. The continual disintegration to which the living tissues are subject, from 

 the various causes already referred to, renders it necessary that a means should 

 be provided for conveying away the waste, as well as for supplying the new ma- 

 terial. This is partly effected by the Venous circulation ; which takes up a 

 large part of the products of incipient decomposition, and conveys them to or- 

 gans of Excretion, by which 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 sys- 

 tem, and the retention of which, therefore, within the body, is the most injuri- 

 ous. Accordingly, we find a most important organ the Pulmonary apparatus 

 adapted to remove it; and to this the whole current of Venous blood passes, 

 before it is again sent through the system. The efficient performance of this 

 function of Respiration is so essential to the well-being of warm-blooded animals, 

 that a special heart is provided for propelling the blood through their lungs, 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, whereby 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, 

 in accordance with the physical law of the mutual diffusion of gases. The in- 

 troduction 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 neces- 

 sary 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 

 nervo-muscular action put forth. The continual formation of carbonic acid, in 

 this and other interstitial changes, has 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, in combination with other combustive processes (in which hy- 



