OF LIFE, AND ITS CONDITIONS. 37 







these animals are subjected, but it has been ascertained that any extraordi- 

 nary act of reparation (such as the reproduction of a limb in the Salamander) 

 will only be performed under the influence of a temperature much higher 

 than that required for the maintenance of the ordinary vital activity. 



18. After the maturity of the Organism has been attained, there is no 

 longer any call for a larger measure of constructive force than is required 

 for the maintenance of its integrity ; and if there were no other source of 

 retrograde metamorphosis than that which is inherent in the peculiar com- 

 position of the tissues, the demand for food would be reduced to very little 

 more than would suffice, by its ultimate conversion into water and carbonic 

 acid, to keep up the Temperature of the body. But the conditions of Animal 

 existence involve a constant expenditure of Motor force through the instru- 

 mentality of the Nervo-muscular apparatus, as well as a liberation of Heat ; 

 and the exercise of the purely Psychical powers through the instrumentality 

 of the Brain constitutes a further expenditure offeree, even when no bodily 

 exertion is made as its result. We have now to consider the conditions 

 under which these forces are developed, and the sources from which they are 

 derived. 



19. That the Motor force which is put forth in the contraction of a Muscle, 

 is generated by the oxygeuation of the component elements, either of the 

 muscle itself, or of the blood which circulates through it, may be considered 

 as demonstrated by these two facts: (1) that while the blood which circu- 

 lates through a Muscle at rest is returned by its veins in an almost arterial 

 condition, its hue changes to venous as soon as the muscle is thrown into 

 action ; and (2) that the quantity of Carbonic acid exhaled bears a constant 

 relation, ca'teris paribiis, to the amount of muscular exertion put forth, as 

 has been especially shown by the experiments of Mr. Newport and Dr. Ed- 

 ward Smith ; the former upon Insects, the latter on the Human subject. It 

 is interesting to note the different ways in which this oxygeuatiou is brought 

 about in the two cases. In the Insect the aeration is quite independent of 

 the blood-circulation ; being provided for by the direct penetration of air, 

 through the ramifications of the trachece, into the substance of the muscles. 

 But in Man as in Vertebrata generally, the Blood is not only the nutrient 

 but is also the oxygenating medium ; and while, in the Organic compounds 

 with which it is charged, it has a store of "potential energy," it is only by 

 the presence of the free Oxygen which it carries with it, that those changes 

 can be maintained whereby "potential energy" is converted into "actual 

 energy." When the doctrine now known as that of the " Conservation of 

 Energy >: l was first worked out in regard to the production of Animal Force, 



1 The earliest distinct general expression of this doctrine is to be found in the very 

 remarkable treatise of Dr. Mayer (Die organische Bewegung in ihren Zusammen- 

 hange mit dem Stoffwechsel, Heilbronn, 1845), in which he worked out from the 

 two fundamental axioms, " Ex nihilo nil ft," and "Nil ft ad nihilnm," the whole 

 system of doctrine which has since come to be known as that of the " Correlation of 

 Forces," and the "Conservation of Energy," in its application alike to Physics and 

 Chemistry and to Physiology. Sir William Grove was simultaneously engaged in 

 the development of the doctrine of the ' Correlation of the Physical Forces " as Prof. 

 Helmholtz was in that of the "Conservation of Energy ;" and without any knowl- 

 edge of the previous labors either of Dr. Mayer or of Prof. Helmholtz. the Author 

 of this Treatise developed a similar doctrine in his Memoir, On the Mutual Rela- 

 tions of the Vital and Physical Forces, published in the Philosophical Transac- 

 tions for 1850. [In a published Introductory Lecture, delivered by Prof. Samuel 

 Jackson, of the University ot Pennsylvania, at the opening of the Session of 1837- 

 1838, he uses these words : " Physical phenomena, according to the class they belong 

 to, are referred to a few simple laws, as gravity, caloric, affinity, galvanism, elec- 

 tricity, magnetism, all of which, it can now be scarcely doubted, are modifications of 

 one great law or force."] 



