376 



MUSCULAR ACTION 



while the cilia and the flagella of spermatozoa are certainly so, although 

 not classed as muscle. Thus, the nervous system, epithelium, the skele- 

 ton, certain parts of the sense-organs, the body-liquids, and the epidermal 

 structures are about the only regions (and these of relatively small 

 volume) in which muscle of some sort is not present. Muscle is the 

 organ or tissue of gross (as distinct from molecular) movement, and but 

 few indeed of the active organs of the body do not need its help. Rather 

 more than, half the mass of the adult human body is muscle, and this is 

 distributed very widely, as we have seen. 



FIG. 228 



FIG. 229 



A muscle-cell from a (nematode) worm, showing 

 an early stage of differentiation. (Claus.) 



Longitudinal partly diagrammatic section 

 in human heart-muscle to show especially the 

 bridges of contractile tissue extending from 

 one cell to the next. (MacCallum.) 



The Structure of Muscle. Like all the other sorts of tissue in the body, 

 muscle is composed of protoplasm arranged (probably because of the 

 modes of production and reproduction) in cells. In the cross-striated 

 muscle-cells, however, differentiation has gone very far, so that the 

 cellular plan, if we think of an average epithelial cell as the type, as is 

 commonly done cannot very readily be made out. In the smooth 

 variety of muscle and in that of the heart the cellular form is more 

 apparent. 



We may divide our brief description of muscle into the three sorts 

 above mentioned: smooth or "unstriated," cross-striated, and cardiac. 

 It seems likely that did we know better the finer structure of muscle these 

 sorts would in part be found to merge histologically as they do physio- 

 logically, for the essential fibril is apparently present in all. 



