HEREDITY OF RACEHORSE STAMINA 41 



dark red. In some fish, the domestic fowl, and in 

 the greater part of the musculature of the rabbit, the 

 muscles are of an extreme pale variety, whilst in the 

 falcon, and other birds which possess the faculty 

 of long-sustained flight, the muscle fibres are almost 

 exclusively dark red. In the horse the two varieties 

 of muscle fibre are found, but not nearly so widely 

 separated in structural appearance as are the muscles 

 of the domestic fowl and the falcon. I shall endea- 

 vour to show, however, that the physiological pro- 

 perties of the two types of muscle are in the horse 

 very widely differentiated, the dark red type being 

 correlated with the capacity for long-sustained 

 muscular effort, whilst the presence of an excess 

 of the pale variety is correlated with inability to 

 respond to repeated stimulation continued for more 

 than a very short period. The difference between 

 the two varieties of muscle fibre may be summarised 

 thus : — 



Pale Red Fibre. 



Dark Red Fibre, 



1. Pale red in colour. 



2. 4 5o*li part of an inch or 



more in diameter, and 

 cross sectional area large 

 in proportion to length. 



3. Transverse stripes very 

 plainly marked, but 

 longitudinal stripes in- 3. 

 distinct. 



Dark red in colour, and in 

 transverse section granu- 

 lar in appearance. 



Less than ^^^oth of an 

 inch in diameter, and 

 cross section small in 

 proportion to length. 



Longitudinal stripes very 

 plainly marked. 



