No. 414.] NORMAL RESPIRATION. 471 
combustion in his laboratory and determines the number of heat 
units liberated ; he can so regulate the process that there shall 
be no by-products, and that no other compounds are included 
in the reaction than those upon which he has determined to 
experiment. In the plant, on the contrary, other substances 
than dextrose may become oxidized, or the oxidation may be 
incomplete. In the laboratory one can deal with measured 
quantities of isolated substances; in the living organism indefi- 
nitely known quantities of many substances together are acted 
upon. Animal physiologists have done much more in this 
direction than have plant physiologists. The higher animals 
are better suited to such studies than are plants. The rela- 
tively high body-temperatures of warm-blooded animals permit 
direct temperature determinations from weighed quantities of 
known foods eaten, as well as calculations from the amounts of 
oxygen needed to effect combustions or decompositions. The 
animal physiologist can check the results obtained by one 
method with those reached through other methods. The 
results obtained by animal physiologists indicate that only about 
95% of the calculated yield of energy from oxidation! appears 
as heat. So we must regard these figures as somewhat too 
high, but their suggestive value is great, whatever must be 
admitted as to their exact numerical value. 
The larger organisms demand for the normal execution of their 
functions more energy than can be supplied by the rearrange- 
ment of the component atoms of the molecules always at 
hand. They must oxidize these molecules, and the more com- 
plete the oxidation, the greater the amount of energy liberated. 
Some of the smaller organisms supply themselves with adequate 
amounts of energy by the destruction of complex compounds 
Within their own living cells. Probably some of the cells of all 
large multicellular organisms have recourse, at times at least, 
to the same means of securing needed energy, and when free 
TEN is not obtainable, the majority of organisms can con- 
at living for a time by so doing. From this the general 
né nce may safely be drawn that the ability to obtain needed 
SY by the destruction of complex substances in the cells 
! See Pembry in Scháfer's Physiology, vol. i, pp. 836, 837. 
