SIM THE NUTRITION OF MUSCLE. [BOOK n. 



material still remaining in the muscular fibres and not to its 

 supplying new raw material. 



Dextrose is, as we have repeatedly said, always present in the 

 blood in small quantity, and appears to be the only carbohydrate 

 constituent of blood-plasma. Experiments carried out on a large 

 animal, such as the horse or cow, have shewn that the venous blood 

 coming from a muscle contains less dextrose than the arterial 

 blood going to the muscle, and that the difference is much 

 increased by throwing the muscle into contraction. From this we 

 may provisionally conclude that dextrose is an essential parr 

 of the food of the muscle. 



The blood as we have seen also contains a certain amount 

 of fat ; and if we push the analogy between the whole body 

 and the muscle we may infer that the muscle takes up fat as food 

 for itself from the blood. But we have no experimental evidence 

 in favour of this. Moreover we have seen that fat and carbo- 

 hydrate are in the animal body more or less transferable. We 

 have distinct proof that the body can transform carbohydrate into 

 4- fat; and it is very probable that it can transform fat into 

 carbohydrate. Seeing how much more easily a soluble diffusible 

 carbohydrate like sugar can be carried from place to place by the 

 fluids of the body than can immiscible fats, it seems reasonable to 

 suppose that when the body has to draw upon its store of fat 

 in the cells of adipose tissue, the fat on leaving the fat cell is 

 transformed into sugar, its carbon so to speak being dealt out to 

 the tissues in the form of dextrose. Indeed we may perhaps, 

 dwelling on the fact that a muscle though itself essentially 

 of proteid build, turns over ( 87) in its daily work so much more 

 carbon than nitrogen, entertain the view that what muscle wants 

 as food is a certain amount of proteid plus an additional quantity 

 of carbon in some form or other, and that dextrose is a convenient 

 form in which the additional carbon can be supplied. And we 

 may hold this view without prejudice to any opinion that the 

 carbon so brought, while being built up into the living substance, 

 may be again arranged as fat, and in the course of the metabolism 

 of the muscle may be later on separated from the living substance 

 and deposited in the fibre as globules of fat. But our knowledge 

 is at present insufficient to decide whether this view is true 

 or no. 



The various salts brought to the muscle by the plasma, though 

 they supply no energy are as essential to the life of muscle as the 

 energy-holding proteid or carbon compound ; and experiments 

 made with regard to some of them, calcic salts for instance, shew 

 that their presence or absence materially affects the maintenance 

 or restoration of irritability. Some of these probably play the 

 part only of securing by their presence favourable conditions for 

 the due metabolic processes, somewhat after the way in which the 

 presence of calcic phosphate determines the curdling of milk ; but 



