MUSCLE. 89 



depends somewhat on the amount of sarcoplasm present. A 

 thin layer of sarcoplasm is usually present under the sarco- 

 lemma. Some authors who regard this as the inner sheath of 

 the sarcolemma call it the endolemma and the outer sheath the 

 epilemma. In the sarcoplasm under the sarcolemma there lie 

 oval nuclei, arranged with their long axes parallel to the long 

 axis of the cell. They are found sometimes in the middle of 

 the fibre, between the primitive fibrils (e. g., in amphibia), but 

 most often are present under the sarcolemma (Fig. 59). In 

 some animals (e. g., the rabbit) we find muscles of two kinds : 

 the so-called red muscle cells, which contain much sarcoplasm 



FIG. 59. 

 A Sarcolemma & 



''' *^ J^X Cohnheim's 



"^ . iis; 7 ft<** 







*' 

 s' \ 



Cross-section of voluntary striated muscle fibres of a rabbit. In A the primitive fibrils 

 are equally distributed ; in B they are grouped into Cohnheim's fields. The fine dots are 

 transverse sections of primitive fibrils. X 1000. 



and possess nuclei in the interior of the cell ; and the white 

 muscle cells, which show a more distinct striation, less sarco- 

 plasm, and nuclei peripherally placed. The more sarcoplasm 

 a cell has, the more slowly will it contract and the longer it will 

 function without tiring. In many vertebrates, and in man 

 also, the muscles are almost exclusively of the red muscle type ; 

 some muscles, however, the so-called mixed muscles, possess both 

 red and white fibres. 



Every fully developed voluntary muscle fibre in the higher 

 vertebrates is surrounded by a sarcolemma. This is a thin, ho- 

 mogeneous, structureless membrane, which under normal condi- 

 tions is so closely approximated to the contents of the cell that 



