HEREDITY OF RACEHORSE STAMINA 39 



action, which, iri its turn, owes its initiation to stimuli 

 passing along the nerve fibres from the central cells 

 in the brain or spinal cord. All muscular tissues 

 have certain common properties, such as excitability, 

 contractility, growth, and respiration. All are made 

 up of units which readily take up oxygen and give 

 off carbon dioxide. But there is, nevertheless, a 

 wide divergence between the muscles which are known 

 as involuntary, or smooth, and found in the walls of 

 the blood vessels and alimentary canal, and those of 

 the heart and the skeletal or voluntary muscle group. 

 The smooth muscles do not concern us here, but, in 

 order to make the subject of stamina in the race- 

 horse intelligible, it is necessary to set forth a brief 

 account of the structure and functions of the unit of 

 voluntary muscle, the muscle fibre or cell. 



In regard to microscopic structure, it cannot be 

 said that finality of opinion has yet been reached. 

 Eminent observers diifer widely as to what is to be 

 seen, and it would be out of place here to enter upon 

 a discussion of the points in dispute. All, however, 

 are agreed that the muscle cell, or fibre, is surrounded 

 by a structureless membrane, the sarcolemma, and 

 that the fibre shows alternate dim and bright trans- 

 verse bands, and can be split up into discs by certain 

 reagents such as osmic acid. It is also agreed that 

 the fibre shows longitudinal striations which appear 

 to separate it into fibrils ranged side by side. " The 

 contents of the muscle fibre appear to consist of two 

 functionally distinct substances, a contractile sub- 

 stance and an interstitial, perhaps nutritive non- 



