GENERAL HISTOLOGY. Ill 



cally, therefore, there are two kinds of muscle, the epithelial 

 muscle-cell and the connective-substance muscle-cell, for the 

 latter of which we have used for a long time the term 

 "contractile fibre-cell." Both kinds of muscle-cells can 

 form a priori smooth as well as cross-striated muscle-sub- 

 stance ; but, generally speaking, the (smooth) musculature 

 of the internal organs arises from the connective tissue-cells, 

 while the epithelial ceils form the (striated) body muscula- 

 ture. 



Fie. 46. Epithelial muscle-cells, a, of a medusa ; 6, of an Actinian. 



Epithelial muscle-cells are cells of which one end ex- 

 tends to the surface of the body or the surface of an inter- 

 nal cavity (body-cavity, lumen of the gut, etc.), and may 

 here have a cuticle, cilia, and flagella, while at the oppo- 

 site end it has secreted contractile substance in the form of 

 muscle-fibrils (Fig. 46). They combine the double func- 

 tion of epithelial and muscle cells. 



Contractile fibre-cells, on the other hand, are connective- 

 tissue cells, which usually have surrounded themselves on 

 all sides with a mantle of contractile substance ; correspond- 

 ing to their origin, they have the form of connective-tissue 

 cells, and are spindle-formed or branched ; in the latter 

 case the branches are functional to the very ends (Fig. 47). 

 The similarity of form renders difficult the distinction be- 

 tween ordinary connective-tissue cells and fibre-cells; if 

 the contractile layer on the surface is slightly developed, 

 the distinction is impossible. To recognize the character 

 of the elements, therefore, we must choose well-defined 

 examples, in which the uninucleated or the multinucleated 

 mass, the " axial substance," is sharply marked off from 

 the muscle-mass, the " enveloping layer" (Fig. 47, e, d, e]. 



In vertebrates and arthropods the contractile fibre-cells 



