86 



MUSCULAR TISSUE. 



[CH VI. 



a soft material called the contractile substance. The sarcolemma 

 is homogeneous, elastic in nature, and especially tough in fish and 

 amphibia. It may readily be demonstrated in a microscopic pre- 

 paration of fresh muscular fibres by applying gentle pressure to 

 the cover slip; the contractile substance is thereby ruptured, 

 leaving the sarcolemma bridging the space (fig. 103). To the 

 sarcolemma are seen adhering some nuclei. 



The contractile substance within the sheath is made up of 

 alternate discs of dark and light substance. 



Muscular fibres contain oval nuclei. In mammalian muscle 

 these are situated just beneath the sarcolemma ; but in frog's 

 muscle they occur also in the thickness of the muscular fibre. 



Fig. 103. Muscular fibre torn across, the 

 sarcolemma still connecting the two 

 parts of the fibre. (Todd and Bow- 

 man.) 



Fig. 104. Muscular fibre of 

 a mammal highly mag- 

 nified. The surface of 

 the fibre is accurately 

 fociissed. (Schafer.) 



The chromoplasm of the nucleus has generally a spiral arrange- 

 ment, and often there is a little granular protoplasm (well seen 

 in the muscular fibres of the diaphragm) around each pole of the 

 nucleus. 



The foregoing facts can be made out with a low power of the 

 microscope ; on examining muscular fibres with a high power 

 other details can be seen. Treatment with different reagents 

 brings out still further points of structure. These are differently 

 described and differently interpreted by different histologists ; and 

 perhaps no subject in the whole of microscopic anatomy has been 

 more keenly debated than the structure of a muscular fibre, and 

 the meaning of the changes that occur when it contracts. A 

 good deal of the difficulty has doubtless arisen from the fact that 

 a muscular fibre is cylindrical, and if one focusses the surface one 

 gets different optical effects from those obtained by focussing deep 

 in the substance of the fibre. I shall, in the following account of 

 the intimate structure of striated muscle, adhere very closely to 

 the writings of Professor Schafer. 



