352 BIOLOGY. [BOOK v. 



co-exists with fibrous contractile elements, which are themselves 

 of varied species. 



According to the strict interpretation of the cellular doctrine 

 generally admitted in Germany, every muscular fibre comes 

 originally from cells, which are simply developed in length by 

 end being joined to end. The nuclei, which we observe here 

 and there along the course of the fibres in the adult, are in that 

 case, the last vestiges of that primitive cellular state. 



Let this be as it may, the typical muscular fibres are of two 

 histological species, connected however with each other by a 

 series of transitory forms. They are smooth or striated. The 

 smooth fibres must be considered in man and the superior verte- 

 brates as fibro-cells very much elongated. In many invertebrates, 

 on the contrary, they seem to be very fine threads without 

 nuclear enlargements and without divisions. It is those fine 

 filiform elements, cylindrical and smooth, which constitute 

 exclusively the muscular apparatus of the inferior invertebrates. 

 Among the mollusks, we already in certain organs, for instance 

 in the pharynx of certain gasteropods, meet with those muscular 

 fibres striated crosswise, which constitute the voluntary muscu- 

 lar apparatus of the arthropods (crustaceans, arachnida, insects) 

 and of the vertebrates. 



This striated muscular apparatus or apparatus of the animal 

 life, resolves itself, in final analysis under the microscope, into very 

 thin fibrils, about the thousandth part of a millimetre in diameter 

 and offering alternately in the direction of their length transparent 

 parts and dark parts of the same extent. These fibrils juxtapose 

 themselves in larger or smaller numbers to form striated muscu- 

 lar bundles, contained in an elastic transparent sheath called 

 sarcolemma or myolemma. In the bundle the fibrils are placed 

 in such fashion, that that there is nearly exact correspondence 

 between their bright parts and their dark parts, whence the 

 striated appearance of the bundle. The volume of these bundles 

 is very variable, according to the animal species, and in the 

 same species according to the regions. It is these bundles which 



