STRUCTURE AND PHYSICAL PROPERTIES OF MUSCLE 531 



they are coarser and coloured red by the presence of a pigment -red 

 muscle. In general, it is found that this difference corresponds to a 

 difference in function. The white muscles are those which are called 

 upon to perform quick movements over short periods, whereas red 

 muscles are those which perform less quick movements, but for longer 

 periods of time, and often without any marked intervals of rest. Thus, 

 the leg muscles of the chicken are red, the breast muscles white; on 

 the other hand, the breast (flying) muscles of the wild-goose are red. 



The length of the fibres also varies greatly with different muscles; 

 from some muscles fibres 12 centimetres long have been obtained. 



FIG. 263. LONGITUDINAL SECTION OF A PIECE OF MUSCLE FROM THE SUCKER CATA- 

 STONIUS. x 1000. THE RELATIONS OF THE DARK AND LIGHT ELEMENTS DURING 

 CONTRACTION OF THE FIBRIL ARE SHOWN IN A, B, C. (After Dahlgren and 

 Kepner.) 



cap., Capillaries; sar., sarcoplasm; mus. n., muscle nuclei; n., connective tissue 

 nuclei: Q, anisotropic material; ?'., non-staining isotropic material. 



Each muscle fibre shows under the microscope longitudinal and 

 transverse striations. The fibres are composed of groups of fibrils 

 (sarcostyles), between which is a varying amount of clear, finely 

 granulated protoplasm the sarcoplasm. Some are rich in sarcostyles 

 and poor in sarcoplasm; others are rich in sarcoplasm, and contain 

 relatively few sarcostyles. In this lies the difference between pale 

 and red muscles. For example, the red soleus muscle of the rabbit 

 contains much sarcoplasm ; the pale gastrocnemius is composed 

 mainly of sarcostyles. The sarcoplasm evidently affords material 

 for sustained action to the sarcostyles, which are the contracting 

 elements. 



