CHEMICAL PHENOMENA OF MUSCULAR CONTRACTION 767 



per second. The maintenance of prolonged tension is, therefore, from 

 the point of view of the mechanical result, an exceedingly wasteful 

 process, with a very low efficiency in comparison with the high efficiency 

 in a rapid twitch. This enables us to see how important a part in heat- 

 production, and therefore in temperature regulation, the tonusof muscle 

 and the prolonged contractions of shivering may possess (p. 695). 



We are as yet in the dark as to the precise relation of the energy 

 which appears as heat and of that which is converted into work. 

 The ultimate source of both is, of course, the oxidation (and 

 cleavage) of the food substances. It was at one time a favourite 

 theory that in a muscle, as in a heat-engine, the chemical energy 

 is first converted into heat, and part of the heat then transformed 

 into work. There is no evidence that this is the case. It is, 

 indeed, impossible that such differences of temperature can exist 

 as would be compatible with the known efficiency of the muscular 

 machine. Hypotheses based on the assumption that the chemical 

 energy is immediately changed into work, perhaps through the pro- 

 duction of surface effects, have met with increasing favour, but 

 data are as yet too few for the formulation of any really satisfactory 

 theory. The close relation between the heat-production and the 

 formation of lactic acid in contraction which have been shown to 

 exist, is a suggestive fact whose full significance will only be revealed 

 by further investigation. The restitution processes by which the 

 original state of the muscle is restored after contraction are, of 

 course, intimately related to those concerned in the actual shorten- 

 ing ; but unless we know how, and in consequence of what chemical 

 or physical changes, the equilibrium of the resting muscle has been 

 disturbed, we cannot know how, or in consequence of what chemical 

 or physical changes, it is restored. 



SECTION IV. CHEMICAL PHENOMENA OF THE MUSCULAR 

 CONTRACTION. 



The composition of dead mammalian muscle of the striped variety may 

 be stated, in round numbers, as follows, but there are considerable 

 variations, even within the same species: 



Water - 75 per cent. 



Proteins - - 20 ,, 



Fats, lecithin, and cholesterin - - 2 



Nitrogenous extractives, creatin (about 0-4 peri 



cent.), carnosin, phospho-carnic acid, inosinic 



acid, purin bodies, such as uric^cid, hypoxan- 



thin, xanthin, etc. - 



Carbo-hydrates (glycogen, dextrose, maltose) 

 Non-nitrogenous organic substances (lactic acid, 



inosit) 

 Pigment (myohaematin or myochrome, a haemoglobin not precisely 



identical with that of blood). 

 Inorganic substances less than i per cent, (chlorides, carbonates, 



phosphates, and sulphates of potassium, sodium, iron, calcium, 



magnesium) . Potassium is absent from the nuclei (Frontispiece). 



