316 J. LeConte on the Correlation of Forces. 
10th. I have spoken thus far of only one source of vital force 
in animals, viz., the decomposition of the tissues. I have attempted 
to show how, upon the principle of conservation of force, this 
is sufficient to carry on the growth and the activity of the ani- 
organism. But decomposition of the tissues, though the 
ee source—the source characteristic of and peculiar to 
mals—of immediate and universal necessity in this kingdom, 
oa im many cases sufficient of itself, is not the only source. 
There is also in animals as in plants a supplemental source, VizZ., 
the iieompdetion 0 of food. 
It is well known that the food - animals consists of two kinds, 
the nitrogenous, such as album n, fibrin, casein, &c., and the 
non-nitrogenous, such as fat, sites sugar, gum, &c. According 
_ to all physiologists since Liebig, the nitrogenous alone are used 
in the repair and growth of ve oe The non-nitrogenous are 
either quickly consumed in respiration, or else are laid up in 
the form of fat for future ou awa in the same way. Now 
there can be no doubt that animals may live entirely 0 on nitro- 
genous food; in which case the whole vital force, whether for 
assimilation or for animal heat and animal activity, is 
derived from the decomposition of the tissues. This is the case 
pe i aap in _ starving animal, particularly if lean. But 
n alm st all cases ood in the form of fat, starch, sugar, 
deo. fon: Sdtropscas)p is never transformed at all into tissues, 
but is taken into the blood, uals decomposed, oxydized in 
the course of the circulation, changed into CO, and HO, and 
finally removed by exhalation from the lungs, Now what is 
the object of the non-nitrogenous food, since these do not form 
any part of the tissues but are again decomposed and thrown 
out of the system? The answer usually given is that such food 
is used in the animal economy solely as fuel to keep up the ani- 
mal heat. On this view it is difficult to see why this class of 
food should be used at all, especially in w But ac- 
fores of aus in the a of dhicaal life generally, yet 
the decomposition of aa en rganic food furnishes ad- 
panes ea by which growth and animal activity may be 
une out too great meptaditens of the tissues. 
