On the structure of cross-striated muscle. 189 



glass plates, the shadow of these upon a screen shows alternate dim 

 and bright stripes. But if now the arrangement be tilted so that the 

 beam of light passes through it somewhat obliquely to the planes of 

 the plates, a dark line makes its appearance in the bright stripe. 

 I mention this experiment, not thereby intending to imply that the 

 arrangement of plates is to be taken strictly as a model of muscular 

 structure, but in illustration of the fact that the appearance of an 

 intermediate line can be optically produced without the actual presence 

 of an intermediate structure. 



Thirdly, the duplicate appearance comes out with the greatest 

 clearness in the acid muscle, and can here always be followed through 

 every focus. The place of the dark dots, represented by Eollett in the 

 middle of the clear stripe, is occupied in every focus merely by a fine 

 transverse line (Figs. 6, 24). From the alteration which the muscular 

 substance has undergone in swelling by the acid, the optical conditions 

 are altered and the former results, shifting with the focus, are not 

 obtained. From this also therefore we may infer that the deep focus 

 appearance of the living and alcohol fibre is illusory, and that the 

 superficial focussing gives the truer image of the muscular structure. 



For the rest it is not easy to understand why Eollett should 

 regard his deep focus appearance (similar to Fig 21 B) with a large 

 dark dot in the place of Z (Zwischenscheibe or intermediate disk of 

 Engelmann, transverse membrane of Krause) as representing the true 

 structural appearance. For it is clear that what will be a deep focus 

 for the superficial plane of the fibre is a high focus for another 

 plane, and so on. If the question were one of high and low focus 

 of a single optical plane of the fibre, there should always be seen, 

 except with the surface foci, a mixture of the deep and superficial 

 focal appearances, which would result in a general blurring of the 

 image. But, of course, this result is not obtained. Nor is the large, 

 dark single intermediate dot obtained in single „fibrils" isolated from 

 alcohol and osmic preparations, (Fig. 23). All that is seen in these 

 ien the „fibril"' is sufficiently extended is a very fine transverse 

 line which may be reduced to a minute dot, but is by no means the 

 conspicuous element of the „fibril" represented in Rollett's figures. Its 



