80 MUSCULAR TISSUE [CH. Vl. 



to the under surface of the skin, or mucous membrane (fig. 102). 

 The fibres in these situations are also finer than in the majority of 

 the voluntary muscles. 



Each fibre consists of a sheath, called the sarcolemma, enclosing 

 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 prepara- 

 tion 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 



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 

 focussed. (Schiifer.) 



occur also in the thickness of the muscular fibre. The chromoplasm 

 of the nucleus has generally a spiral arrangement, 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 structure of 



