652 THE NUTRITION OF MUSCLE. [Book ii. 



words ' directly ' and k indirectly ' used in this connection. 

 And we may here repeat the caution (§ 30) that though for 

 convenience sake we use the phrase ' living substance, ' what 

 is really meant by the Words is not a thing or body of a par- 

 ticular chemical composition but matter undergoing a series of 

 changes. 



435. We know more about the chemical changes of muscle 

 than perhaps of any other tissue, though this even at the most 

 is not much, and we may perhaps take the nutrition of muscle 

 as a type of nutrition in general. The muscle in a normal state 

 of things lives ultimately on the proteids, fats, carbohydrates, 

 salts and water of the food, and on the oxygen of the inspired 

 air, but lives directly on the blood which brings these things 

 to it. 



Concerning the relation of proteids to muscle, we know 

 little more than was stated in § 140 in speaking of the heart. 

 We can do no more than infer, and that doubtfully, that serum- 

 albumin is the form in which the muscle takes up proteids. 

 Concerning carbohydrates we have apparently a definite fact. 

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

 the blood in small quantity, and appears to be the only carbo- 

 hydrate 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 dif- 

 ference is much increased by throwing the muscle into contrac- 

 tion. From this we may provisionally conclude that dextrose 

 is an essential part of the food of the muscle. 



Concerning fats we have little or no knowledge, but we 

 may perhaps infer that the body has power to transform fats 

 into carbohydrates as it has the power to transform carbohy- 

 drates into fats, and that the carbon whether of the fat or of 

 the carbohydrates of food is presented to the muscle in the 

 form of carbohydrate, namely of dextrose. But we have no 

 distinct proof of this. 



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, some- 

 what after the way in which the presence of a calcic salt deter- 

 mines the clotting of blood and the curdling of milk; but some 

 we probably ought to regard as actually entering into the pro- 

 cesses themselves. Of these matters however we know very 

 little. 



