MUSCULAR ACTION 



381 



are termed sarcostyles. Cross-striated muscle is well named, therefore, 

 for numerous striations in both directions divide the protoplasm into 

 many sorts of minute cubical compartments or units whose respective 

 significance is not yet made out either structurally or functionally. 

 The histology of muscle is still further complicated by the complex 

 optical properties of the substance, so that the appearances under one 

 set of conditions are quite unlike those in the same structure under 

 another set, while preserving and staining methods produce very vari- 

 ous effects. It is possible, as MacCallum suggests, that the optical 

 appearance of this sort of muscle is partly due to a reticulum with regu- 

 lar meshes extending through the protoplasm, of which, the ultimate 

 fibrils and Krause's membrane, so called, are the chief elements. (See 

 Fig. 278 page 499.) 



According to a common notion, in the middle of each of these sarcomeric 

 parts, and transversely across it, is Krause's "membrane;" on either side 

 of it an intermediary disk of clear isotropic substance (sarcoplasm or 



FIG. 237 



FIG. 238 



r-C 



Cross-section in human heart-muscle, above 

 or else below the nucleus: C, central sarco- 

 plasm-mass; S, sarcoplasmic disk; F, fibril- 

 bundle. (MacCallum.) 



Longitudinal section in human heart-muscle: 

 S, sarcoplasmic disks; F, fibril-bundle; K, 

 Krause's membrane. (MacCallum.) 



lymph) ; next a broad dark band which is bisected by Hensen's median 

 (light) disk, and finally this in turn by the median (dark) disk of Heiden- 

 hain. Krause's "membrane" appears to extend across the fiber and to 

 be continuous with the sarcolemma. An observation chance made by 

 Kiihne seems to place the consistence of the crossing-structures still 

 more in doubt than before. If really membranous, Krause's "mem- 

 brane" is of only slightly greater consistence than its surroundings. 



It is rather more than possible that the individual fibers are connected 

 by some sort or other of bridges made of muscular tissue. 



The nuclei of the cells of striated muscles of man are mostly just within 

 the sarcolemma, but sometimes, as in the red sort of rabbit-muscle, deep 

 within the fiber-cell. About the nuclei is often a more or less granular 

 substance. 



The vascular and nervous service of muscle is very abundant, the 



