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 oxyhsemoglobin, 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 occurs in the tissues, and especially in the 

 muscles. It was found by Pfliiger, 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 

 Giirber 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 frequently 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. Chem., Strassburg, 1891, Bd. xv. S. 387; Araki, ibid., 

 1891-4, Bde. xv., xvi., xvii., xix. 



2 The disappearance of the oxygen of oxy haemoglobin 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 oxyhsemoglobin 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 oxyhsemoglobin hermetically sealed in glass tubes. 



3 Arch.f. d. ges. Physiol., Bonn, Bd. x. S. 251 ; see also (Ertmann, ibid., 1877, Bd. xv. 

 S. 382. 



4 Journ. Physiol., Cambridge and London, 1894, vol. xv. p. 449. 



6 In connection with this question, the possibility must not be forgotten that even 

 ordinary proteids may have a carbohydrate nucleus in their molecule (cf. p. 64). 



