228 GENERAL VIEW OF THE FUNCTIONS. 



agency of the Nervous system ; and are therefore to be referred to the class 

 of Organic movements, such as occur in Vegetables. 



273. Upon the circulation of the blood through all parts of the fabric, de- 

 pends, in the first place, the Nutrition of the tissues. Upon this subject, for- 

 merly involved in the greatest obscurity, much light has recently been thrown 

 by Microscopic discovery ; it being now understood (as explained in the 

 preceding Chapter), that the continued growth and renewal of each tissue are 

 effected by a continuation of a process of cell-growth, similar to that by which 

 it was first developed. Even where the primary cells have changed their 

 character, their nuclei remain persistent; and may be regarded (in the lan- 

 guage of Mr. J. Goodsir) as so many "germinal centres," for giving origin to 

 new products. The greatest difficulty, in the present condition of our know- 

 ledge of this most interesting subject, is to comprehend the reason why such a 

 variety of products should spring up ; when the cells in which they all origi- 

 nate, appear to be so exactly alike. The important discoveries now referred 

 to are not confined to healthy structures ; for it has been ascertained, that dis- 

 eased growths have a similar origin and mode of extension ; and that the ma- 

 lignant character, assigned to Cancer, Fungus Haematodes, and other such 

 productions, is to be traced to 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 Fungus, a large structure may 

 rapidly spring up, the removal of which is by no means attended with any 

 certainty that it will not speedily re-appear, from some germs left in the sys- 

 tem. 



274. The independent character of the cells in which all organized tissues 

 originate, might be of itself a satisfactory proof that, in Animals, as in Plants, 

 the actions of Nutrition are performed by the powers with which they are 

 individually endowed ; and that, whatever influence the Nervous system may 

 have upon them, they are not in any way essentially dependent upon it. More- 

 over, there is an evident improbability in the idea, " that any one of the solid 

 textures of the living body should have for its office, to give to any other the 

 power of taking on any vital actions :" and the improbability becomes an im- 

 possibility, 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 Nerv- 

 ous System is known to have upon the Function of Nutrition, is probably ex- 

 erted, 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 influence, 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. 



275. 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 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 system, and the retention of which, therefore, within the 



