HEREDITY OF RACEHORSE STAMINA 43 



and, secondly, that waste products are formed by 

 the muscle faster than they can be removed. It is, 

 of course, true that fatigue does not wholly depend 

 on the lodgment in the muscle fibre oi carbon dioxide 

 and other broken down products, for experiments 

 clearly show that these substances also act on the 

 central and peripheral nerve tissues ; but in the 

 racehorse, muscle asphyxiation is the primary cause. 



The muscles in the heavy breeds of horse appear 

 to be exclusively of a pale red variety. The shire 

 horse, even when thoroughly conditioned and fit, 

 rapidly suffers from muscle asphyxiation if made to 

 gallop at his top pace, poor as it is, for a short distance. 

 In some individuals a furlong will suffice to produce 

 distress. 



In the thoroughbred both varieties of muscle are 

 to be found in the same individual, and, although in 

 this class of horse the opportunities for making 

 microscopic examinations are few, it is permissible 

 to conclude from the specimens of muscles which I 

 have examined that the dark red fibres greatly pre- 

 ponderate in the muscles of a " stayer " and the 

 paler variety in the muscles of a " sprinter."* At 

 all events, I trust it will be clear from the evidence 

 which I shall advance that the physiological proper- 

 ties of the muscles of certain racehorses are sharply 



* Mr. Robertson tells me, in answer to my inquiry, that he thinks 

 about 75 per cent, of the muscles of a true "sprinter" are of the pale 

 variety. In an " Intermediate " racer he thinks there is about half and 

 half, and in a "stayer,'' .such as Willonyx,he would sa3' that 90 per 

 cent, of the fibres are of the deep red variety'. These statements are of 

 course very approximate, and they partake of the nature of constructing 

 the whole horse from a microscopic examination of a few muscles 

 onlv. — Editor. 



