THE PHYSIOLOGY OF MTSCULAR WORK 21 '> 



bodies. It is impossible to give a satisfactory account of their 

 significance in the physiology of muscular work, but an attempt 

 will be made to pick out the observations of most interest. 



Creatin is a substance which crystallises out when an aqueous 

 extract of meat is allowed to evaporate. It has the formula 

 C 4 H tt N 3 2 . E. Mellanby () has found that it is present in 

 different amounts in the muscles of various animals ; as a general 

 rule there is a greater quantity in the muscles of animals in an 

 ascending scale of development from the cold-blooded to the 

 warm-blooded. This is illustrated by the following table : 



Hedgehog (winter) . 

 (summer) 

 Rats (two months old) 

 Bullock . 



Pig. 

 Rabbits . 



2 per cent. 



2 



3 



3 



33 



44 



A further interesting difference was discovered by an analysis 

 of the muscles of similar animals during several stages of de- 

 velopment. 



Rabbits (fatal, 21 days) 



7 days old . 



9 

 12 

 19 

 25 

 39 

 46 

 adult 



Chick embryo, before 12th day 



weight 6-1 grm.s. 12th day . 

 11-3 14th . 

 17-3 16th . 

 26-0 18th . 

 30-25 20th . 



Chick, weight 36-0 gnus. 1 day after hatching, but 

 no food taken . ... 



a trace. 



191 per cent. 



228 



283 



316 



300 



390 



373 



435 



no trace. 



a trace. 



. 3-8 mgre. total. 

 6'6 

 . 11-8 

 . 13-25 



230 



In the embryo chick creatin is absent at a stage of develop- 

 ment when muscle is present ; in hedgehogs the quantity of 

 creatin in the muscles remains constant throughout the year, 

 notwithstanding the great differences in the metabolism of these 

 animals during the periods of hibernation and activity. 



