896 METABOLISM. 
and the greater the amount of muscular activity the greater the amount 
of oxidised materials in the form of carbonic acid and water that are 
formed and got rid of from the body. It is probable that the oxidation . 
processes which occur in gland cells are by no means so active, for 
although a gland when stimulated to activity receives a larger amount 
of oxygenated blood, yet a considerable amount of the oxygen of that 
blood simply passes through the capillaries without being absorbed, so 
much so, in fact, that, as noted by Bernard, the blood of the veins of the 
salivary glands during stimulation of their cranial nerves flows almost 
as bright red as that of an artery. And in confirmation of this we find 
that the largest gland in the body, the liver, is supplied with a relatively 
small amount of arterial blood, and that almost the whole of its 
metabolic activity is carried on with blood which already has passed 
through the intestinal capillaries, and which has thereby been deprived 
of a large part of its oxygen. Further, it was noted by Ludwig that the 
saliva flowing from the duct of the submaxillary gland contains more 
oxygen, than is dissolved in the plasma of the arterial blood, an 
indication that the cells of the salivary glands cannot be greedy of 
oxygen since they pass oxygen out along with the secretion rather than 
retaining it for the formation of carbon dioxide and water. The 
salivary glands, moreover, have been shown by the recent careful 
observations of Bayliss and Hill x not to produce any appreciable amount 
of heat ; and although it is stated that the blood flowing through the 
liver is the warmest blood in the body, 2 and warmer than that flowing 
through the muscles, it must be borne in mind that it is almost 
impossible to measure exactly the normal temperature of the blood 
flowing from the muscles, because the operation necessary for observing 
the temperature of such blood would tend to expose it to loss of heat. 3 
In conformity with the conclusion that the muscles are the organs which 
possess by far the greatest amount of metabolic activity, it has been estimated 
that the muscular tissues contain about one-fourth of the whole blood of the 
body. The liver, which has important special functions to perform in 
metabolism — functions which are, however, probably in large measure inde- 
pendent of oxidation — contains another fourth of the blood, one-fourth is 
employed in keeping full the larger arteries and veins, and all the rest of the 
body put together has for its capillary supply only the remaining fourth. It 
is clear, then, that in all observations and experiments upon the metabolism 
of the body, the metabolism of the muscles must occupy a prominent place. 
Nitrogenous Metabolism in the Tissues. 
Of the proteids of the body Voit distinguishes two kinds — (1) Those 
which form an integral part of the living substance or bioplasm, and (2) 
those which occur in the tissue juice and in contact with the bioplasm, 
but which are not to be regarded as forming an integral part of that 
substance itself. To this latter kind he has given the name of " circu- 
1 Joum. Physiol., Cambridge aud London, 1S94, vol. xvi. p. 351. Previously to this 
work, it had been accepted, on the authority of Ludwig and others, that the secretion of 
saliva is accompanied by a marked production of heat within the submaxillary gland. 
2 See p. 826. Waymouth Reid was unable to find any effect on the temperature of the 
liver as the result of stimulating the splanchnic and vagi nerves ("Proc. Phys. Soc," 
1895, p. xxxi., in Joum. Physiol., Cambridge and London, vol. xviii. ). 
3 In his experiments upon the gaseous exchange in blood perfused through "surviving " 
mammalian muscle, v. Frey found that the blood leaving the muscle was slightly warmer 
than that entering it {Arch. f. Physiol., Leipzig, 1885, p. 559). 
