GENERAL HISTOLOGY 81 



Smooth and Cross-striated Muscle Fibres. The smooth muscle- 

 substance represents a lower stage than the cross-striated, since it chiefly 

 occurs in the less highly organized and more inactive animals. Interest- 

 ing in this respect is the fact that in the two stages of development of the 

 same animal the simple and inert polyp has smooth muscles, while the 

 more highly organized and actively motile medusa has cross-striated 

 muscles (fig. 49). The difference in their action has led in the verte- 

 brates to a peculiar distribution of the muscle-substance, the smooth 

 musculature being chiefly in internal organs, which are not under control 

 of the will (Involuntary muscles] , while the musculature of the body, subject 



a. 



FIG. 49. Epithelial muscle-cells, a, of a medusa; b, of an actinian. 



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to the will and hence demanding more rapid action, is cross-striated 

 (voluntary muscles}. We must not conclude that the difference between 

 smooth and cross-striated musculature coincides with the distinction be- 

 tween visceral and body musculature; it should be noticed that the body 

 musculature of all molluscs is smooth, the visceral as well as the body 

 muscles of many insects and Crustacea, and the muscles of the heart of 

 vertebrates are cross-striated. 



It was pointed out above, in connection with epithelia and connective 

 tissue, that these tissues differed fundamentally. This contrast has its 

 bearing in dealing with the muscles, for both epithelial and mesenchy- 

 matous cells may form contractile substances and therefore there are two 

 genetically different kinds of muscles, the epithelial and the mesenchy- 

 matous (contractile fibre-cell). Both kinds of muscle-cells can a priori 

 form smooth as well as cross-striated muscle-substance; but the collection 

 of connective (mesenchymatous) tissue around inner organs has caused 

 most contractile fibre-cells to be smooth, while most of the epithelial 

 muscle-cells are cross-striated. 



Epithelial muscle-cells are cells of which one end extends to the surface 

 of the body or the surface of an internal cavity (body cavity, lumen of the 

 gut, etc.), and may here have a cuticle, cilia, or flagella, while at the op- 

 posite end it has secreted contractile substance in the form of muscle- 

 fibrils (fig. 49). They combine the double function of epithelial and 



muscle cells. 

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