82 



GENERAL TRIXCIPLES OF ZOOLOGY 



Conlraclilc Jihrc-cells, on the other hand, are eonneetive-tissue cells, 

 ■which usually ha\e surrounded themselves with a layer of contractile 

 substance; corresponding to their origin, they have the form of connective- 

 tissue cells, and are spindle-formed or branched; the branches arising from 

 the enils of the cells (lig. 50). The similarity of form renders the distinc- 

 tion between ordinar}- connective-tissue cells and fibre-cells dithcult; 

 if the contractile layer on the surface be slightly developed, the distinction 



Fig. 50. 



Fig. 50. — Contractile libre-cells. a, of man; b-c, of Error (a Clonophorc); />, young 

 fibres; r, branched ends; (/, middle portion of a libre; f, cross-section. 



Fio. 51. — Cross-striated primary bundle (after CegenliaurV //, nuclei; .■;, a point 

 where the sarcolcmma is plainh" shown on accouiU of the tearing c>f the libres. 



is impossible. To recognize the character of the elements, therefore, 

 we must choose well-tlelined examples, in which the nucleatetl mass, the 

 'axial substance,' is sharply marked off from the muscle-mass, the 'cortical 

 layer' (Irg. 50, r, </, <■). The regular arrangement of the epithelial cells 

 side l>v side, gi\'es the tnuscle lilires arising from them a parallel arrange- 

 ment, so that a layer is formeil, which ])ecomes folded when much muscle 



