26 PHYSIOLOGY CHAP. 



lemma, but they belong to the muscle substance, and not to the 

 sarcolemma. Each muscle fibre may be regarded as a very 

 elongated cell, provided with several nuclei, the sarcolemma 

 representing the cell membrane. The diameter of the fibres 

 usually varies from 30 to 40 /*, but may be greater or less in 

 different classes of animals. 



The substance proper or protoplasm of the fibre presents a 

 double striation, longitudinal and transverse, owing to the fact 

 that it consists of a bundle of numerous primitive fibrils arranged 

 parallel, each of which has a complex transverse structure. 



On examining a fibre in cross-section, each primitive fibril 

 appears as a rounded spherical granule, comparatively dark in 

 colour, surrounded by a lighter non-differentiated substance the 

 sarcoplasm. The amount of sarcoplasm may vary considerably in 



FJG. 18. Tranverse section of two striated muscle fibres of rabbit. (Szymonowicz.) Magnified 

 1000 diameters. At A the primitive fibrils (S) are equally distributed in the sarcoplasm (Sp). 

 At B they form polyhedric segments known as Cohnheim's areas (Cc). 



different muscles, just as the mode of division or grouping of the 

 primitive fibrils within the sarcoplasm also differs (Fig. 18). 



On examination of a fresh muscle fibre in serum, a longi- 

 tudinal striation is seen owing to the parallel arrangement of the 

 primitive fibrillae. A series of light and dark striae at right 

 angles to the longitudinal axis of the fibre are also visible, which 

 are due to a double series of light and dark parallel bands that 

 alternate regularly through the entire length of the fibre. The 

 dark striae are broader than the light, and show at the boundary 

 of the clear bands a darker layer which seems to consist of a series 

 of dots. 



On teasing out dead muscle fibres hardened in alcohol, it is 

 possible to separate the primitive fibrils. This is easiest in 

 animals which have the most abundant sarcoplasm (Fig. 20). 

 Under a high power, each fibril is seen to consist of alternating 

 light and dark bands of approximately uniform width. But in 

 the middle of the clear band there is a very fine dark line, 



