864 METABOLISM AND STRUCTURE. [BOOK n. 



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, cf. 162, 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 

 some we probably ought to regard as actually entering into the 

 processes themselves. Of these matters however we know very 

 little. 



543. The end-products of muscular metabolism are as we 

 have seen carbonic acid, lactic acid, and kreatin or some other 

 nitrogenous bodies, and we have already ( 87) said all we have 

 to say concerning the formation of these products. We may how- 

 ever briefly consider here the question, What is the relation of 

 these various metabolic processes to the structural elements of 

 the tissue ? When we say that the muscular fibre is continually 

 undergoing metabolism do we mean that every jot and tittle of 

 the fibre is undergoing change and that at the same rate ? We 

 can hardly suppose this. It seems unlikely, for instance, that the 

 metabolism of the fibrillar substance is identical with that of the 

 interfibrillar substance, whatever be the view we take as to the 

 properties or meaning of the two substances. Further, if we 

 accept the suggestions made in 87 as to a contractile substance, 

 which, though having peculiar qualities, being peculiarly related 

 to and having peculiar connections with the rest of the fibre, may 

 in a broad way be compared with the glycogen of a hepatic cell, 

 we can conceive that this contractile substance may be manufac- 

 tured without the whole of it at least having been at any time an 

 integral part of what we may in a stricter sense call the real living 

 substance of the fibre. We should thus be led to regard the 

 metabolic events occurring in muscle as falling into two classes at 

 least ; those taking place in the living more permanent framework, 

 and those bearing on the formation and destruction of the con- 

 tractile substance lodged in that living framework. Further, if 

 we suppose that the metabolism by which the muscles supply 

 so much of the heat of the body, and which as we have seen 

 may and does go on independently of contractions, is not a 

 metabolism of the same contractile substance differing from the 

 metabolism of a contraction in being so ordered that all the 

 energy goes out as heat, none being employed to effect a change 

 of form, but is a metabolism of some other ' thermogenic ' sub- 



