72 GENERAL VIEW OF THE FUNCTIONS. 



77. Indeed it appears to be principally for the maintenance of these instru- 

 ments in a state fit for activity, that by far the largest part of the food ingested 

 by most Animals is required. * The duration of the existence of the Muscular 

 and Nervous tissues, appears to depend entirely upon the use that is made of 

 them ; being less as their functional activity is greater. Thus, when an ani- 

 mal is very inactive, it requires but little nutrition ; if in moderate activity, 

 there is a moderate demand for food ; but if its muscular energy be frequently 

 and powerfully aroused, the supply must be increased, in order to maintain 

 the vigour of the system. There would seem reason, then, to believe, that 

 every animal movement, requiring the expenditure of a certain degree of mus- 

 cular power, does really involve the death and re-formation of a certain amount 

 of muscular tissue ; and this idea is confirmed by the fact, that the quantity of 

 waste thrown off by the excreting processes, or, at least, of that part of it 

 which results from the disintegration of the muscular structure, is propor- 

 tional to the expenditure of muscular power, increasing (like the demand for 

 food which is consequent upon it) with the general activity, and diminishing 

 with rest. This doctrine, which was first pointedly stated by Liebig, though 

 propounded in more general terms by previous writers, may probably be ex- 

 tended from the Muscular system (in regard to which alone it has been urged 

 by Liebig) to the Nervous, as well as to the various organs of Nutrition. Many 

 circumstances lead to the belief, that the Nervous tissue, when in a state of 

 functional activity, undergoes a rapid waste or disintegration, and a correspond- 

 ing renewal. The very large quantity of blood with which the nervous cen- 

 tres are supplied, and the immediate dependence of nervous power upon the 

 maintenance of that supply ( 177), strongly indicate this; for we invariablj 

 find, that parts which undergo little interstitial change, receive but a small 

 supply of blood. Again, it is well known that, when the nervous system has 

 been in unusual activity, there is a marked increase in the phosphatic depo- 

 sits in the urine ; and, as the quantity of phosphorus in any others of the soft 

 tissues is very inconsiderable, it is scarcely possible to attribute this liberation 

 of phosphorus from the system to any other cause than the waste of nervous 

 matter, that is, its decomposition, resulting from the discharge of its vital 

 function. Again, the close chemical relation which subsists between nervous 

 and adipose matter (the substance peculiar to nervous tissue being a fatty acid, 

 containing a very small proportion of azote, but united with a considerable 

 amount of phosphorus,) corresponds exactly with the old observation, that per- 

 sons of " nervous temperament" are seldom fat ; whilst those of inert bodily 

 and mental habits are much more subject to this deposit. Since nervous 

 matter is chiefly formed out of the same elements as those which would other- 

 wise be deposited as adipose tissue, it appears probable that the demand for 

 these, occasioned by the continual use of the nervous system, prevents the de- 

 posit of fat ; whilst its inactivity allows their accumulation in another form. 

 It may probably be stated, then, as a general ,truth, 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. 



78. The degree in which the operations of the mind are dependent upon its 

 material instruments, is a question which cannot be regarded as conclusively 

 determined by scientific evidence alone ; and it has little practical bearing on 

 Physiological research. The doctrine usually regarded as having the best 

 Scriptural basis, that the mind has an existence altogether distinct from that 

 of the body, is attended with several difficulties, of which those arising dut of 

 the phenomena of Insanity are perhaps the most important. On the other hand, 

 the opinion held by some, that mental phenomena are the mere results of ma- 

 terial changes, appears to involve difficulties at least equal ; amongst which 



\ 



