32 



ELECTRO-PHYSIOLOGY 



CHAP. 



again in a minority in the same parts of the goose and pigeon, 

 while they are wholly wanting in the falcon, crow, and sparrow 

 (Fig. 21). 



From birds onwards the dark fibres are usually in the ascend- 

 ant. According to Rollett the elements of the skeletal muscles 

 of the bat are peculiarly rich in sarcoplasm. In transverse 

 section it appears as a number of coarse, irregular knots, drawn 

 to one side or the other, and united by fine, slender bridges of 

 protoplasm ; the muscle-columns lie in the intermediate spaces 



(Fig. 22, ). 



The superficial aspect of the fibres is correspondingly coarse, 



a 



iRittliilii 



Ifffifilttffj 



[ilillllMMi 



FIG. 22. Transverse and longitudinal sections of the muscle-fibre of Bat. Sarcoplasm clear, 

 fibril-bundles (muscle-columns) dark. (Rollett.) 



with longitudinal striae, due to the alternation of sarcoplasm and 

 muscle-columns (Fig. 22, I). In most other mammals the dark 

 and relatively plasmic fibres are intermixed with clear fibres in 

 the skeletal muscles. Knoll finds that the optic, masticatory, 

 and respiratory muscles (in particular the diaphragm) are specially 

 rich in dark fibres. In almost all vertebrates the dark are 

 smaller than the clear fibres. This is well shown in transverse 

 sections of muscles containing both kinds of fibres (pectoral 

 muscle of pigeon, most muscles of amphibia and mammals) (Fig. 

 21). Moreover, the "dark" fibres in many cases are charac- 

 terised by their deep red colour, while the "clear" fibres are 

 paler. The dark lateral muscles of many fish, e.f/., and the float 

 Mud cardiac muscles, are intensely red; in Rana cscul. and temp. 

 also, the muscles of the throat and heart are dark and red. 



