MUSCLE 209 



of the tissue. (For the chemistry of the carbohydrates, see 

 p. 285.) 



(2) Fat is present in small quantities in the fibres, and 

 often in very considerable quantities in the fibrous tissue 

 between them, especially in some animals, e.g. the pig. 



(3) Inosite, formerly called muscle sugar, is present in 

 small amounts. It is an isomere of glucose CgHjoOg, but it 

 is a cyclic compound and not a carbohydrate. 



(4) Sarcolactic Acid. — Hydroxy-propionic acid — 



H OH O 



I I II 

 H— C— C— C— 0— H 



I ! 



H H 



This dextro-rotatory isomere of ordinary lactic acid is 

 hardly to be detected in resting muscle when the latter is 

 rapidly removed and at once placed in ice-cold alcohol. But 

 it is rapidly formed in excised muscle, and it is markedly 

 increased both in muscle and in the urine by hard exercise 

 and by mal- oxygenation. Along with COg and NaHjPO^ it 

 plays a part in causing the phenomena of fatigue. 



(5) From muscle, after removal of the proteins, a 

 series of bodies containing nitrogen may be extracted. 

 The chief of these is Creatin, methyl-guanidin-acetic acid. 

 Guanidin C.NH(N 112)3 i^ ^ '^sar ally of urea CO(NH2)2. 



Methyl-guanidin is produced by replacing an H in 

 guanidin by CHg, and in creatin this is linked to acetic 

 acid — 



H O 



H— N CHg 

 H,N— C— N- 



Methyl-guanidin 



_C— C— OH 



I 

 H acetic acid 



There is some evidence that methyl-guanidin may exist 

 as such in muscle. 



The amount of creatin in the muscles of various species 

 of animals is very constant. In man about 0-4 per cent, 

 may be extracted. Some maintain that it does not exist 

 14 



