ON THE BALANCE OF THE VITAL ECONOMY. 437 



that under such circumstances, either little mental work would be done, or 

 more food would be required. Supposing only seven miles were walked, 

 118 foot-tons of force would be required, leaving 236 foot-tons available for 

 mental work ; but it must be remembered that both mental work and much 

 of the internal mechanical work are again expended in the body. A part 

 of the mechanical force exerted by the heart is given up as heat from ob- 

 structed motion in the capillaries, and will be included in the heat given off 

 from the body ; and although nothing positive can be stated as to the form 

 taken by mental work and nervous action generally, it is highly probable 

 that for" the most part this is heat. There is every reason to believe that, 

 although the tissue of the nerves and muscles cannot be built up from ali- 

 mentary materials destitute of nitrogen, the force which can be exerted by 

 those tissues proceeds both from the disintegration and oxidation of nitro- 

 genous as well as non-nitrogenous substances. At any rate, it has been 

 clearly shown by Professor Haughton that the amount of urea (501 grains) 

 eliminated after severe exercise does not represent the oxidation of a suffi- 

 cient amount of nitrogenous material to account for the work done. . More- 

 over, whilst in carnivorous animals the force must proceed from the oxida- 

 tion of nitrogenous constituents, since they can be nourished upon lean flesh, 

 in herbivorous animals and, as Verloren has pointed out, in mature insects, 

 as bees and butterflies it seems equally evident that the nervo-muscular 

 force is generated from the oxidation of farinaceous compounds ; and an 

 examination of the excreta gives additional testimony to the same effect. 

 Thus, Professor Haughtou found the daily elimination of urea in a Bengal 

 tiger to be 4375 grains, whilst in a sheep it was only 256 grains. It has 

 also been shown by the same observer that in diabetes, where sugar is elimi- 

 nated by the urine, instead of undergoing combustion and being discharged 

 from the body in the form of carbonic acid and water, an increased quantity 

 of food is required, which is accompanied by an increased discharge of urea, 

 showing that the nitrogenous materials are consumed to maintain the tem- 

 perature and to furnish the force required for work, which is always small. 

 Heatou suggests that the whole, or nearly the whole, of the force of the 

 body is generated in the blood, and that Mayer was perfectly right in saying 

 that the muscle produces mechanical effect at the expense of the chemical 

 action occurring in its capillary vessels. It is probable, however, that the 

 oxidizing processes take place essentially in the tissues and iutertextural 

 fluids. 



350. Of the mode in which the substitution of new tissue for that which 

 has become effete, is effected in the process of Nutrition, our knowledge is 

 at present limited; but there can be little doubt that it nearly always takes 

 place in a manner closely conformable to the first development of each tissue. 

 In some instances there is an obvious replacement of the old and dead by the 

 young and active elements: this is the case, for example, in the constantly 

 repeated production of the Epidermic and Epithelial layers; for whether 

 they are developed from germs imbedded in the subjacent basement-mem- 

 brane, or from nuclei formed de novo in the blastema on its free surface, or 

 by the duplicative subdivision of pre-existing cells, there is a continual suc- 

 cession of new cells, which take the place of those that are cast off as de- 

 funct and useless. So in the growth of Hair, the increase of which takes 

 place only at its base, we can trace at any period the same development of 

 newly-formed spheroidal cells into horny fusiform fibres, as that which oc- 

 curred when first the hair began to sprout from the aggregation of epidermic 

 cells at the bottom of its follicle. So, again, in the vesicular tissue which 

 constitutes the essential part of the Nervous centres, there are appearances 

 which indicate that its peculiar cells are in a state of continual development, 



