42 



THE MENDEL JOURNAL 



Pale Red Fibre. 



Dark Red Fibre. 



Owing to tlie bulk of the 

 fibres in cross section, 

 the capillary network of 

 blood vessels running be- 

 tween them is narrow. 



Stimulation repeated at 

 short intervals rapidly 

 produces exhaustion. 



Moderate stimulation is 

 sufficient to produce a 

 maximum contraction. 



In the living animal, the 

 essential constituents of 

 live muscular substance 

 are rapidly broken down 

 during continued muscu- 

 lar contraction. Carbon 

 dioxide, together with 

 other end products col- 

 lect in the muscle, pro- 

 ducing the phenomena 

 of tissue asphyxiation 

 and fatigue. 



The capillary network ex- 

 ceeds in size the ordinary 

 capillaries, and is dilated 

 into little reservoirs. 



Stimulation at short in- 

 tervals does not rapidly 

 produce exhaustion. 



Requires powerful stimu- 

 lus to ensure a maximum 

 contraction. 



In the living animal, the 

 essential constituents of 

 the live muscle appear to 

 possess an accentuated 

 faculty of storing up 

 oxygen, and perhaps gly- 

 cogen, in some com- 

 pound or compounds. 

 The amount of carbon 

 dioxide given off by the 

 muscle during work is at 

 the same time compara- 

 tively small, and passes 

 immediately into the 

 dilated capillary loops, 

 in place of lodging in the 

 muscle fibre itself. Tissue 

 asphyxiation and fatigue 

 is, consequently, long 

 delayed. 



From the above it will be gathered that, in the 

 case of a horse whose muscles are principally, if not 

 solely, of the pale red variety, the materials necessary 

 for contraction are used up during continued fast 

 work much more quickly than they can be renewed, 



