182 TIIK MORPHOLOGY OF THE COMMON CllAYFISH. 



lighter stria?, transversely to the axis of the fibres 

 (fig. 52 A). The distance of the transverse strife from one 

 another varies -with the condition of the muscle, from 

 1 -4,000th of an inch in the quiescent state to as little as 

 1 -30,000th of an inch in that of extreme contraction. 

 The more delicate muscular fibres, like those of the 

 heart and those of the intestine, are imbedded in tlie 

 connective tissue of the organ, but have no special sheath; . 



Fig. 53. — A,ifaciis fliiv'uifiUs. — .1, living muscular fibres very liij,^hly 

 magnified ; B, a fibrilla treated with solution of sodium chloride ; 

 C, a fibrilla treated with strong nitric acid, s, septal lines ; sz, 

 septal zones ; is, interseptal zones ; a, transverss line in the inter- 

 septal zone. 



The fibres which make up the more conspicuous muscles 

 of the trunk and limbs, on the other hand, are much 

 larger, and are invested by a thin, transparent, structure- 

 less sheath, which is termed the sarcolemma. Nuclei 

 are scattered, at intervals, through the striated substance 

 of the muscle ; and, in the larger muscular fibres, a layer 

 of nucleated protojjlasm lies between the sarcolemma and 

 the striated muscle substance. 



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