222 GENERAL VIEW OF THE FUNCTIONS. 



things, in which the Animal powers are much developed, an almost entire 

 want of that tendency to indefinite extension, which is so characteristic of the 

 Plant; and when the large amount of food consumed by them is considered, 

 the question naturally arises, to what purpose this food is applied, and what 

 is the necessity for the continued activity of the Organic functions, when once 

 the fabric has attained the limit of its development. 



263. The answer to this question lies in the fact, that the exercise of the 

 Animal functions is essentially destructive of their instruments ; every ope- 

 ration of the Nervous and Muscular systems requiring, as its necessary con- 

 dition, a disintegration of a certain part of their tissues, probably by their ele- 

 ments being caused to unite with oxygen. The duration of the existence of 

 those tissues (as stated in the preceding Chapter) varies inversely to the use 

 that is made of them ; being less as their functional activity is greater. Hence, 

 when an Animal is very inactive, it requires but little nutrition ; if in mode- 

 rate activity, there is a moderate demand for food ; but if its Nervous and 

 Muscular energy be frequently and powerfully aroused, the supply must be 

 increased, in order to maintain the vigour of the system. In like manner, the 

 amount of certain products of excretion, which result from the disintegration 

 of the Nervous and Muscular tissues, increases with their activity, and dimin- 

 ishes in proportion to their freedom from exertion.* We are not to measure 

 the activity of the Nervous system, however, like that of the Muscular, only 

 by the amount of movement to which it gives origin. For there is equal evi- 

 dence, that the demand for blood in the brain, the amount of nutrition it re- 

 ceives, and the degree of disintegration it undergoes, are proportional likewise 

 to the energy of the purely psychical operations ; so that the vigorous exercise 

 of the intellectual powers, or a long-continued state of agitation of the feelings, 

 produces as great a waste of Nervous matter, as is occasioned by active bodily 

 exercise. From this and other considerations, we are almost irresistibly led 

 to the belief, that every act of Mind is inseparably connected, in our present 

 state of being, with material changes in the Nervous System; a doctrine not 

 in the least inconsistent with the belief in the separate immaterial existence of 

 the Mind itself, nor with the expectation of a future state, in which the com- 

 munion of Mind with Mind shall be more direct and unfettered. 



264. Thus in the Animal fabric, among the higher classes at least, the func- 

 tion or purpose of the organs of Vegetative life is not so much the extension 

 of the fabric, for this has certain definite limits, as the maintenance of its in- 

 tegrity, by the reparation of the destructive effects of the exercise of the 

 purely Animal powers. Thus, by the operations of Digestion, Assimilation, 

 and Circulation, the nutritious materials are prepared and conveyed to the 

 points where they are required ; the Circulation of Blood also serves to convey 

 oxygen, which is introduced by the Respiratory process ; and it has further 

 for its office to convey away the products of the decomposition of the Muscular 

 and Nervous tissues that results from their functional activity, these products 

 being destined to be separated by the Respiratory and other Excreting opera- 

 tions. In the performance of the Organic functions of Animals, as in those 

 of Plants, there is a continual new production, decay, exuviation, and renewal, 

 of the cells, by whose instrumentality they are effected ; which altogether effect 

 a change not less complete than of the leaves in Plants. But it takes place in 

 the penetralia of the system, in such a manner as to elude observation, except 

 that of the most scrutinizing kind; and it has been in bringing this into view, 

 that the Microscope has rendered most essential service in Physiology. 



* This doctrine, though propounded in general terms by previous writers, was first point- 

 edly stated by Prof. Liubig, so far as regards Muscular tissue, iu his Treatise on Animal 

 Chemistry. It will be hereafter shown, however, to be equally applicable to the Nervous 

 substance. 



