NORMAL HISTOLOGY. 



62 



lemma; in the majority of other vertebrates, however, the nuclei 

 are distributed irregularly throughout all parts of the contractile 

 substance. These differences are well shown in the accompanying 

 figures. 



The muscle -fibres present alternafe light and dark transverse 

 markings, or striae, to which the tissuejowes its characteristic appear- 

 ance. The highly specialized 

 contents of the sarcolemma are 

 composed of two substances pos- 

 sessing different refractive prop- 

 erties, that forming the dark 

 bands being doubly refracting, 

 or anisotropic, while that of 





FIG. 72. 



Fibres of voluntary muscle in section : A , 

 human fibres, with nuclei upon the surface and 

 beneath the sarcolemma ; B, fibres from frog, with 

 nuclei embedded within the muscle-substance. 



Voluntary muscle, portions of two fibres show- 

 ing the characteristic transverse markings; the 

 lighter band is divided by the row of minute beads 

 constituting the intermediate disk : a, termination 

 of muscular substance and attachment of adjoin- 

 ing fibrous tissue ; n, nuclei of muscle-fibres. 



the light striae is singly refracting, or isotropic. When fresh or 

 well-preserved mammalian muscle is examined under high am- 

 plification it is seen that the dark striae, or transverse disks, are 

 not unbroken homogeneous bands, but that each is composed of a 

 number of minute prismatic elements placed side by side and sep- 

 arated from one another by a thin layer of a substance corresponding 

 to and continuous with that forming the light zone. This latter, in 

 addition, is divided transversely by a delicate interrupted line or 

 row of dark dots the intermediate disk, or membrane of Krause. 

 That part of the light zone between the dim intermediate and trans- 

 verse disks constitutes the lateral disk. 



The explanation of these appearances has caused many and pro- 

 longed discussions, and even at present, notwithstanding the careful 

 study bestowed upon the subject, the exact structure of voluntary 

 muscle must be regarded as still unsettled. Heretofore two promi- 

 nent and opposed views have prevailed : the one regards the fibre as 

 composed of parallel longitudinal rows of minute prisms forming 

 fibrillae (as rows of bricks placed end to end); the other considers 

 the fibre as built up by the apposition of their disks, whose diameter 

 corresponds to that of the entire fibre (as cheese-boxes piled one 



