88 THE CONTRACTILE TISSUES. 



in the connective tissue between the fibres, there is also present in the 

 muscular substance within the sarcolemma, always some and at times 

 a great deal of fat, chiefly ordinary fat, viz., stearin, palmitiri, and olein 

 in variable proportion, but also the more complex fat lecithin. As to 

 the function of these several fats in the life of the muscle we know little 

 or nothing 



Carbohydrates, the third of the three great classes in which we may group 

 the energy-holding substances of which the animal body and its food are 

 alike composed, viz., proteids, fat, and carbohydrates, are represented in 

 muscle by a peculiar body, glycogen, which we shall have to study in detail 

 later on. We must here merely say that glycogen is a body closely allied 

 to starch, having a formula, which may be included under the general 

 formula for starches, x (C 6 H, O 5 ), and may like it be converted by the action 

 of acids, or by the action of particular ferments known as amylolytic fer- 

 ments, into some form of sugar, dextrose (C 6 H, 2 O 6 ), or some allied sugar. 

 Many, if not all, living muscles contain a certain amount, and some, under 

 certain circumstances, a considerable amount, of glycogen. During or after 

 rigor mortis this glycogen is very apt to be converted into dextrose, or an 

 allied sugar. The muscles of the embryo at an early stage contain a rela- 

 tively enormous quantity of glycogen, a fact which suggests that the glyco- 

 gen of muscle is carbohydrate food of the muscle about to be wrought up 

 into the living muscular substance. 



The bodies which we have called extractives are numerous and 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 mus- 

 cular 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 

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

 on, far exceeds in importance all the other nitrogenous extractives of the 

 body as a whole, since it is practically the one form in which nitrogenous 

 wastes leave 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 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 is, in normal condi- 

 tions, present in muscular substance either living and irritable or dead and 

 rigid ; urea does not arise in muscular substance itself as one of the imme- 

 diate waste products of muscular substance. 



There is, however, always present in relatively considerable amount, on 

 an average about 0.25 per cent, 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 substituted 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 sur- 

 plus of carbon and hydrogen arranged as a body belonging to the fatty acid 

 series. We shall have to return to this kreatin and consider its relation to 

 urea and to muscle when we come to deal with urine. 



The other nitrogenous extractives, such as karnin, hypoxanthin (or 



