CHAP, ii.] THE CONTRACTILE TISSUES. 91 



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 fibrillae. Their existence seems to 

 indicate that the fibrillae are arranged in longitudinal prisms 

 separated from each other by a larger amount of interfibrillar 

 substance than that uniting together the individual fibrillae 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 fibrillcc (probably cylindrical in general form and probably 

 arranged in longitudinal prisms) imbedded in an interfibrillar 

 substance, which appears to be less differentiated than the fibrillae 

 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 fibrillae, and 

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

 with longitudinal spaces corresponding to the fibrillae. 



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

 fibrillae, 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 fibrillae and interfibrillar substance having different 

 refractive powers, some of the optical features of muscle may be due, 

 on the one hand to the relative proportion of fibrillae to inter- 

 fibrillar substance, and on the other hand to the fibrillae not being 

 cylindrical throughout the length of the fibre but constricted at 

 intervals, and thus becoming beaded or moniliform ; for instance 

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

 corresponding to aggregations of interfibrillar material filling up 

 the spaces where the fibrillae are most constricted. But it does 

 not seem possible at the present time to make any statement which 

 will satisfactorily explain all the various appearances met with. 



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

 when a contraction wave sweeps over the fibre ? 



Muscular fibres may be examined even under high powers of the 

 microscope while they are yet living and contractile ; the contraction 

 itself may be seen, but the rate at which the wave travels is too 



