138 THE TISSUES. 



the fibrils (M. Heidenhain). By grouping the unequally refracting 

 substances (or unequally staining substances) a fibril may be divided 

 into successive portions or protoplasmic metameres which may be 

 termed sarcomeres (Schafer) and which are bounded by the mem- 

 brane of Krause (z). In such a sarcomere or muscle-casket we 

 may recognize, beginning with Krause's membrane, z, an isotropic 

 intermediary disc, j ; an anisotropic, transverse disc, Q, divided by 

 a less refracting Hensen's disc, H, into two equal parts, Plensen's 

 disc showing the median membrane of Heidenhain, M ; again an iso- 



Fig. 96. Diagram of the 

 structure of the fibrils of a stri- 

 ated muscle-fiber. The light 

 spaces between the fibrils may 

 represent the sarcoplasm. 



P'ig. 97. Diagrams of the transverse stria- 

 tion in the muscle of an arthropod ; to the right 

 with the objective above, to the left with the ob- 

 jective below its normal focal distance (after Rol- 

 lett, 85 ): Q, Transverse disc ; h, median disc 

 (Hensen) ; , terminal disc (Merkel) ; A 7 , acces- 

 sory disc (Engelmann) ; J, isotropic substance. 



tropic intermedian disc, j, and Krause's membrane, z. Krause's 

 membrane, as above stated, is continuous across the small bundles 

 into which the fibrils are grouped, and is also attached to the sar- 

 colemma (M. Heidenhain, J. B. MacCallum). This is shown to the 

 left in Fig. 96, where the sarcolemma appears festooned, with 

 Krause's membrane attached, thus indicating clearly the sarcomeres. 

 One of the best objects for the study of transverse striation is 

 the muscle of some of the arthropods (beetles). In the striated 



