DEPENDENCE OF THE VITAL ACTIONS. 361 



active powers of locomotion, but a much greater variety of Animal faculties ; 

 and the instruments of the Organic or nutritive operations attain their highest 

 development, and their greatest degree of mutual dependence. We see in the 

 fabric of all beings, 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. 



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

 Animal functions is essentially destructive of their instruments; every operation 

 of the Nervous and Muscular systems involving, as its necessary condition, a 

 disintegration of a certain part of their tissues ; so that the duration of the ex- 

 istence of those tissues varies inversely to the use that is made of them, being 

 less as their functional activity is greater. Hence, when an Animal is very in- 

 active, it requires but little nutrition ; if in moderate activity, there is a moderate 

 demand for food ; but if its Nervo-muscular energy be frequently and powerfully 

 aroused, the supply must be increased, in order to maintain the vigor of the 

 system. 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 evidence, that the demand for blood in the Brain, 

 the amount of nutrition it receives, and the degree of disintegration it under- 

 goes, 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 communion of Mind with Mind shall be more direct and 

 unfettered. 



371. 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 integrity, 

 by the reparation of the destructive effects of the exercise of the purely Animal 

 powers. By the operations of Digestion, Assimilation, and Circulation, the 

 nutritive materials are prepared and conveyed to the points where they are re- 

 quired ; the Circulation of Blood also serves to transmit oxygen, which is intro- 

 duced by the Respiratory process ; and it has further for its office to convey away 

 the products of that decomposition of the Muscular and Nervous tissues, which 

 results from their functional activity, these products being destined to be sepa- 

 rated by the Respiratory and other Excreting operations. 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 instru- 

 mentality they are effected ; which altogether effect a change not less complete 

 than that 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 scru- 

 tinizing kind ; and it has been in bringing this into view, that the Microscope 

 has rendered most essential service in Physiology. 



372. The regular maintenance of the functions of Animal life is thus entirely 

 dependent upon the due performance of the Nutritive operations; a considera- 

 tion of great importance in practice, since a very large proportion of what are 

 termed " functional disorders" (of the Nervous system especially) are immedi- 

 ately dependent upon some abnormal condition of the Blood. But there also 



