CHAP, ii.] THE CONTRACTILE TISSUES. 93 



fresh living fibre. These areas are spoken of as " Cohnheim's 

 areas ; " they are very much larger than the diameter of a fibrilla 

 as indicated by the longitudinal striation, and indeed correspond 

 to a whole bundle of such fibrillse. Their existence seems to 

 indicate that the fibrillse are arranged in longitudinal prisms 

 separated from each other by a larger amount of interfibrillar 

 substance than that uniting together the individual fibrillse form- 

 ing each prism. 



Lastly, it may be mentioned that not only are the various 

 granular lines at times visible with difficulty or quite invisible, but 

 that even the distinction between dim and bright bands is on 

 occasion very faint or obscure, the whole muscle-substance, apart 

 from the nuclei, appearing almost homogeneous. 



Without attempting to discuss the many and various interpre- 

 tations of the above and other details concerning the minute 

 structure of striated muscular fibre, we may here content ourselves 

 with the following general conclusions. 



(1) That the muscle-substance is composed of longitudinally 

 disposed fibrillce (probably cylindrical in general form and probably 

 arranged in longitudinal prisms) imbedded in an interfibrillar 

 substance, which appears to be less differentiated than the fibrillse 

 themselves and which is probably continuous with the undifferen- 

 tiated protoplasm round the nuclei. The interfibrillar substance 

 stains more readily with gold chloride than do the fibril Ia3, and 

 hence in gold chloride specimens appears as a sort of meshwork, 

 with longitudinal spaces corresponding to the fibrillse. 



(2) That the interfibrillar substance is, relatively to the 

 fibrillse, more abundant in the muscles of some animals than in 

 those of others, being for instance very conspicuous in the muscles 

 of insects, in which animals we should naturally expect the less 

 differentiated material to be more plentiful than in the muscles of 

 the more highly developed mammal. 



(3) That, the fibrillse and interfibrillar substance having dif- 

 ferent refractive powers, some of the optical features of muscle may 

 be due, on the one hand to the relative proportion of fibrillse to 

 interfibrillar substance, and on the other hand to the fibrillse not 

 being cylindrical throughout the length of the fibre but constricted 

 at intervals, and thus becoming beaded or moriiliform. For in- 

 stance the rows of granules spoken of above are by some regarded 

 as corresponding to aggregations of interfibrillar material filling 

 up the spaces where the fibrillse are most constricted. And, 

 indeed, some authors maintain that the whole striation is simply 

 an optical effect due to the disposition of the surface of the fibre. 

 It does not seem possible' at the present time to make any 

 statement which will satisfactorily explain all the various appear- 

 ances met with. 



55. We may now return to the question, What happens 

 when a contraction wave sweeps over the fibre ? 



