216 THE PHYSIOLOGY OF MUSCULAR WORK 



Creatin is absent from invertebrate muscle, although morpho- 

 logically and physiologically the cross striated muscle of these 

 animals is identical with that of vertebrates. The closely related 

 substance creatinin, C 4 H r N 3 0, is not present in muscle, and is 

 not formed from creatin during prolonged work. Creatin has 

 apparently no influence on muscular contraction or the passage of 

 a nervous impulse into muscle. From a consideration of these and 

 other facts, Mellanby suggests that creatinin is formed into creatin 

 and -stored in the muscle, and that the development of the liver 

 in the vertebrates may account for the presence of creatin in their 

 muscles ; the gland of the mid-gut of invertebrates, although it 

 has been called a liver by some biologists, has no morphological or 

 physiological connection with the liver of vertebrates. The urine 

 of young children is almost free from creatinin, and chicks do not 

 excrete creatinin until about a week after hatching, by which time 

 their muscles are saturated with creatin. 



The other nitrogenous extractives found in muscle are carnic 

 acid, C 10 H 15 N 3 5 , combined with phosphorus to form phospho- 

 carnic acid ; carnosin, C 9 H 14 N 4 3 ; inosinic add, C 10 H 13 N 4 ?0 8 ; 

 small quantities of urea, CO(NH 2 ) 2 ; and of the following purine 

 bodies hypoxanthine or oxypurine, C 5 H 4 N 4 ; xanthine or dioxy- 

 purine, C 5 H 4 N 4 2 ; and uric acid or trioxypurine, C & H 4 N 4 3 . The 

 significance of these substances in the metabolism of muscle is 

 unknown. 



THE INORGANIC SALTS OF MUSCLE 



There are considerable variations in the quantity of the in- 

 organic salts which are present in the muscles of different animals, 

 but in all the samples analysed potassium and phosphorus are the 

 most abundant constituents. The following results expressed in 

 parts per 1000 were obtained by Bunge ( 10 ) : 



