i ORGANISATION AND STRUCTURE OF MUSCLE 33 



The striated muscles of mammals are mostly red, and (with 

 the exception of the cardiac muscle-cells, and the muscles of the 

 bat, which are uniformly dark) contain a mixture of clear and 

 dark fibres. 



The variations of colour in different muscles are particularly 

 striking in the domestic animals. Every one knows the " white 

 meat" of breast and back in the common fowl, which consists 

 exclusively of clear fibres, while the leg-muscles are red and 

 dark in colour. In tame rabbits and guinea-pigs also, white pale 

 muscles occur along with the strictly red of the heart, diaphragm, 

 etc. ; and here again dark fibres preponderate in the one case, and 

 clear in the other, but the degree of colour and quantity of proto- 

 plasm are not always proportional. Thus in the pigeon the red 

 wing-muscles are composed chiefly of dark, the equally red leg- 

 muscles of clear fibres, and in the rabbit also there is little differ- 

 ence in amount of protoplasm between the dark red soleus and the 

 pale adductor magnus muscles. In Triton and Salamandra, the leg- 

 muscles are reddish, but distinctly dark in single fibres only ; the 

 rest of the muscles are pale and clear. Eanvier also finds a dis- 

 tinction in number and distribution of the nuclei between red 

 and pale muscles, but, according to Knoll, no general rule can be laid 

 down. In all mammalian muscles investigated by him, the nuclei 

 are "predominantly" set round the periphery of the fibre, but there 

 are always individual fibres with instanding nuclei, and this not 

 only in the red muscles as stated by Eauvier, but in the definitely 

 pale muscles also (adductor magnus of rabbit). Generally speak- 

 ing, the centrally-situated nucleus corresponds with a low grade of 

 development in muscle-fibre, and is therefore the rule only in fishes 

 and amphibia, in which last many nuclei are often strung together 

 in a longitudinal series, while in the muscle-fibres of birds and 

 mammals the nuclei are generally placed at the periphery, close 

 under the sarcolemma ; but here again there may be exceptions. 



Within the sarcoplasm, which, as follows from its develop- 

 ment, represents the original formative plasma, lie the structures 

 described by Kolliker as " interstitial granules," and these, in 

 virtue of their strong refractibility, are, when accumulated between 

 the bundles of fibrils, the cause of the " dark," opaque properties 

 of certain muscle-fibres. 



The greatest variety of mass-disposition, and relative propor- 

 tion of sarcoplasm and fibrils, is exhibited by the striated muscle- 



D 



