CHAP, v.] NUTRITION. 823 



concerned in the breaking down (katabolic) of the living substance. 

 We shall explain presently what we mean by the words, ' directly ' 

 and '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 particular chemical composition but matter 

 undergoing a series of changes. 



542. Since the several tissues originate through a differ- 

 entiation of the simpler, primordial protoplasm, we may infer that 

 we have a right to speak of a general plan of metabolism common 

 to all the tissues, modified in various particulars in various tissues. 

 It is more reasonable for instance to suppose that there is such a 

 general plan common to both muscle and gland, than to suppose 

 that the metabolism of the one differs wholly from or only acci- 

 dentally resembles that of the other. And we may profitably take 

 the nutrition of muscle as exemplifying, in the midst of the features 

 special to muscle, the general plan of vital metabolism. 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. Taking the proteids first we may ask the 

 question, How does the blood supply the muscle with proteids ? 



The blood contains three classes of proteids: (1) serum- 

 albumin, (2) globulin (paraglobulin), and (3) fibrinogen, that is to 

 say, the body or bodies concerned in the clotting of blood, whose 

 nature we left in 23 as not wholly and clearly made out. With 

 regard to the function of these three kinds of proteids in the 

 nutrition of muscle our only conclusions at present are indirect 

 ones, based chiefly on the results of experiments as to the 

 relative value of these substances in maintaining or restoring the 

 irritability of muscle. It is found that when the washed out 

 frog's heart ( 162) is fed with defibrinated blood, the restoration 

 is as good as with whole blood ; and that while the effects of 

 globulin are uncertain, and while peptone and albumose appear to 

 act in an injurious manner, the restorative effects of serum-albumin 

 are marked. From these results we may provisionally infer that 

 the muscle in its (total) anabolic changes takes up and so lives 

 upon the serum-albumin of the blood. But this conclusion must 

 be regarded as provisional only, and indeed uncertain. For we 

 must remember that the blood supplies not only the food 

 (including oxygen) for the muscle, but also the conditions under 

 which the muscle can live and avail itself of the food offered to it. 

 The complex actions through which a certain quantity of proteid 

 and other material is built up into living muscular substance need 

 for their execution a favourable medium, need certain physical 

 and chemical conditions ; and it may be that the favourable 

 influence of serum-albumin is simply due to its presence in some 

 way assisting the transformation into living substance of raw 



