MUSCULAR TISSUE. 



139 



muscle of beetles and other arthropods there is, however, a further 

 division into isotropic and anisotropic substance. Here it will be 

 noticed that the disc j is separated by an anisotropic disc, known as 

 the accessory disc of Engelman, and designated by the letter N, into 

 an isotropic disc j, next to the anisotropic transverse disc Q, and an 

 isotropic disc, known as Merkel's terminal disc, and designated by 

 the letter E, situated next to Krause's membrane (z). (See lower 

 portion of Fig. 96.) The muscle fibrils present a different appear- 

 ance when focused high than they do when focused low, as may be 

 seen from the diagram given in Fig. 97 ; those parts which appear 

 light on high focusing appear dark on deep focusing. 



Sarcoplasm. 



Cohnheim's 

 area. 



Sarcolemma. 



Sarcoplasm. 

 Fibrils. 



Sarcolemma. 



Fig. 98. Transverse section through striated muscle-fibers of a rabbit. I and 3, 

 from a muscle of the lower extremity ; 2, from a lingual muscle ; X 9 CO - In 2 > Cohn- 

 heim's fields are distinct; in I, less clearly shown ; in 3, the muscle-fibrils are more 

 evenly distributed. 



It has recently been suggested by J. B. MacCallum that Krause's 

 membrane with the primitive fibrils form a continuous network in 

 the muscie-fiber, the meshes of which would be fairly regular, the 

 fibrils of such a network which run parallel to the long axis of the 

 muscle-fiber being larger than the cross fibrils. Such a network is 

 not to be confused with a network which may be brought out on 

 staining striated muscle-fibers with gold chlorid, which network is 

 due, in part at least, to a staining of the sarcoplasm. 



The ultimate fibrils are grouped into small bundles (0.3-0.5 // in 



