CHAP, ii.] THE CONTRACTILE TISSUES. 93 



varied. They are especially interesting since it seems probable 

 that they are waste products of the metabolism of the muscular 

 substance, and the study of them may be expected to throw light 

 on the chemical change which muscular substance undergoes during 

 life. Since, as we shall see, muscular substance forms by far the 

 greater part of the nitrogenous that is, proteid portion of the 

 body, the nitrogenous extractives of muscle demand peculiar atten- 

 tion. Now, the body urea, which we shall have to study in detail 

 later on, far exceeds in importance all the other nitrogenous extrac- 

 tives of the body as a whole, since it is practically the one form in 

 which nitrogenous waste leaves the body ; if we include with urea, 

 the closely allied uric acid (which for present purposes may simply 

 be regarded as a variety of urea), we may say broadly that all the 

 nitrogen taken in as food sooner or later leaves the body as urea ; 

 compared with this all other nitrogenous waste thrown out from 

 the body is insignificant. Of the urea which thus leaves the body, 

 a considerable portion must at some time or other have existed, or, 

 to speak more exactly, its nitrogen must have existed as the nitrogen 

 of the proteids of muscular substance. Nevertheless, no urea at all 

 or an absolutely minimal quantity only is, in normal conditions, 

 present in muscular substance either living and irritable, or dead 

 and rigid ; urea does not arise in muscular substance itself as one 

 of the immediate waste products of muscular substance. 



There is, however, always present, in relatively considerable 

 amount, on an average about *25 p.c. of wet muscle, a remarkable 

 body, kreatin. This is in one sense a compound of urea : it may 

 be split up into urea and sarcosin. This latter body is a methyl 

 glycin, that is to say, a glycin in which methyl has been sub- 

 stituted for hydrogen, and glycin itself is amido-acetic acid, a 

 compound of amidogen, that is a representative of ammonia, and 

 acetic acid. Hence kreatin contains urea, which has close relations 

 with ammonia, together with another representative of ammonia, 

 and a surplus of carbon and hydrogen arranged as a body belonging 

 to the fatty acid series. We shall have to return to this kreatin, 

 and to consider its relations to urea and to muscle, when we come 

 to deal with urine. 



The other nitrogenous extractives, such as karnin, hypoxanthin 

 (or sarkin), xanthin, taurin, &c., occur in small quantity, and need 

 not be dwelt on here. 



Among non-nitrogenous extractives, the most important is the 

 sarcolactic acid, of which we have already spoken ; to this may 

 be added sugar in some form or other, either coming from glycogen 

 or from some other source. 



The ash of muscle, like the ash of the blood corpuscles, and, 

 indeed, the ash of the tissues in general, as distinguished from the 

 blood, or plasma, or lymph on which the tissues live, is character- 

 ised by the preponderance of potassium salts and of phosphates ; 

 these form in fact nearly 80 p.c. of the whole ash. 



