On the Relations of the isotropous to the auisotropous Layers in striped Muscles. 537 



From a long and careful study of the best attainable microscopic 

 images of Crustacean muscle in all stages of contraction, and from 

 a comparison of my own results with those recorded by Engelmann, 

 KöLLiKER, Krause, Merkel, Retzius, Rollet, Schäfer and others, 

 I am convinced that the isotropous layer in some way disappears into 

 the anisotropous. The anisotropous substance must then be either, 

 1) an unorganised mass, capable on stimulation of free mixture of its 

 substance with the isotropous substance, and this seems capable of 

 explaining the phenomena visible in Crustacean muscles (cf. the figs, 

 and the diagram 24) ; or 2) it presents a definite structure in adaptation 

 to greater physiological activity, the isotropous substance no more 

 mixing freely with the anisotropous substance as a whole, but flowing 

 rapidly in and out of longitudinal pits. This discovery by Schäfer 

 of the longitudinal pits in the anisotropous layer in the most rapidly 

 contracting and expanding muscles known to us seems to me by its 

 obvious specialisation in the interests of rapidity, to throw a flood of 

 light on the real nature of muscle contraction. 



IL 



The next point was ; "Is the attraction of the anisotropous layers 

 for the isotropous the essential factor in muscle contraction" ? Confining 

 myself entirely to Crustacean muscle, I made a great many measurements 

 always along the same fibrils or along fibrils of the same muscle 0^ 

 in order to ascertain whether the fully contracted "element" was shorter 

 than the anisotropous layer in the expanded „element", i. e. to ascertain 

 whether the anisotropous layer remained the same length after the 

 merging of the isotropous layer in it. A little longer and a little 

 thicker it might be, owing to the absorption of so much new substance, 

 but materially shorter it ought not to be^). This indeed is what I 



1) It was also necessary to confine the measurements to the middle 

 portion of muscles, as near the ends, the "elements" were often irregular 

 in length. Fig. 23 is a drawing to scale of the extreme end of a fibril. 



2) The assumption that the anisotropous layer shortens on getting 

 thicker, seems to me to rest entirely on the very artificial supposition 

 that the fibril is provided with membranous elastic hoops around these 

 layers. It is obvious that if the membranous investment of the iso- 

 tropous layers (that is, if any such exist) were equally elastic, no con- 

 traction could take place in the manner described , for the isotropous 

 layers would lengthen in the same proportion as the anisotropous shorten. 

 As a matter of fact, the existence of any investing membrane has yet 

 to be proved. What Schäfer speaks of as a membrane, as revealed by 

 his photographs, seems to refer to the outer ring of the anisotropous 

 substance itself, rather than to a true investing membrane. 



