METABOLIC ACTIVITY OF TISSUES AND ORGANS. 895 
metabolism respectively, to appear in larger amount than usual in the 
blood, and to become excreted in the urine. 1 
Eelative Metabolic Activity of the Tissues and Organs. 
Before we trace the fate of the foodstuffs in the body, it is 
important we should have an idea of the relative metabolic activity of 
the tissues, since all essential changes which contribute to the pro- 
duction of the energy of the body occur within the tissues. 
It was at one time believed that the blood was the seat of 
important oxidation processes ; but whilst it cannot be denied that a 
certain amount of oxidation may occur in the blood, as shown by the 
rapid diminution in the oxygen of the oxyhemoglobin, on allowing blood 
to stand in a closed vessel, 2 it is certain that by far the greatest part of 
the oxidations in the body 00010-3 in the tissues, and especially in the 
muscles. It was found by Pmiger, that frogs whose blood had been 
wholly replaced by salt solution took in just as much oxygen, and gave 
off just as much C0 2 , as normal animals. 3 Moreover, Pembrey and 
Griirber found hardly any alteration in the oxidation processes in rabbits 
which had been deprived of a large proportion of their blood. 4 
Placing the tissues in order of relative activity, the muscles must 
take the first place ; next to these the secreting glands ; and next to 
these the tissues of the nervous system, especially the grey matter. 
Last in the scale come the skeletal tissues, which, performing as they 
do a passive function, may be assumed to exhibit comparatively little 
metabolic activity. With regard to the most active of the tissues, 
namely, the muscles and the cells of secreting glands, we may note, in 
passing, that their chemical composition is by no means identical. 
The most prominent organic material in muscular tissue is native 
proteid of the globulin class, whereas the most prominent organic 
materials in the living tissue of gland cells are nucleo-proteids. This 
distinction, though frecpiiently ignored, is one of considerable import- 
ance, for the nucleo-proteids have a constitution more complex than 
that of proteids, consisting as they do of a combination of proteid with 
phosphorus-containing substances, which yield as products of decomposi- 
tion, xanthine bases, nucleins, paranucleins, and phosphoric acid, and 
some of them, at all events, a carbohydrate (see pp. 66, 67). 5 
There can be very little doubt that the greater part of the oxidation 
of the body occurs in the muscles. The formation of heat can, in fact, 
be shown to be mainly due to the chemical activity of the muscles, an 
activity called into play under the influence of the nervous system ; 
1 Zillesen, Ztschr. f. physioL Ohem., Strassburg, 1391, Bd. xv. S. 387; Araki, ibid. , 
1891-4, Bde. xv., xvi., xvii., xix. 
2 The disappearance of the oxygen of oxyhemoglobin which occurs in blood on standing, 
has been ascribed to the presence of hypothetical substances, to which the term "reducing 
substances " has been applied. No chemical substances having such a reducing power have, 
however, been either isolated from blood or chemically investigated. Moreover, the reduc- 
tion of oxyhemoglobin in blood on standing, may be due to its oxygen being removed 
by the bioplasm both of the white corpuscles and of putrefactive bacteria, which rapidly 
begin to appear and multiply in drawn blood. Reduction even occurs with solutions of 
pure crystallized oxyhemoglobin hermetically sealed in glass tubes. 
3 Arch. f. d. ges. Physiol., Bonn, Bd. x. S. 251 ; see also CErtmann, ibid., 1877, Bd. xv. 
S. 382. 
4 Journ. Physiol., Cambridge and London, 1894, vol. xv. p. 449. 
6 In connection with this cpiestion, the possibility must not be forgotten that even 
ordinaiy proteids may have a carbohydrate nucleus in their molecule (cf. p. 64). 
