GENERAL PHYSIOLOGY OF MUSCLE 



27 



which was first described by Aniici, but is generally known as 

 Krause's membrane. Krause regarded this as a delicate little 

 membrane, dividing the fibrils into a series of segments which he 

 called sarcomeres. In the muscles of certain insects as well as 

 those of some mammals, a further differentiation is visible with 

 strong magnification in both light and dark bands, for the 

 description of which the reader must refer to text-books of modern 

 histology. 



From the physiological point of view the different refracting 



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FIG. 19. (Left.) Muscular fibre of a mammal examined fresh in .serum. (Schiifer.) Highly 

 magnified. 



FIG. 20. (Right.) Fragment of frog's muscle fibre in which a few fibres have been isolated. 

 (Szymonowicz.) Magnified about 650 diameters, n, nucleus ; fp, primitive fibril ; is, 

 isotropous layer; an, anisotropous layer; A, Amici's striae or intermediate disc. 



power of the respective light or dark bands of muscle fibres is 

 more important. Boeck of Christiania was the first who pointed 

 out that certain tissues, among them the muscles, were doubly 

 refracting or anisotropous, but Briicke (1857) showed that the 

 whole fibre is not anisotropous, a portion of its substance being 

 singly refracting or isotropous. When the fibres are viewed by 

 polarised light, the dark striae show up light on the black ground 

 formed by crossed Nicol prisms : the light striae, on the contrary, 

 appear dark. The former are doubly, the latter singly refracting. 

 To obtain a clear idea of the changes which the striatiou of 

 the muscle fibre undergoes during contraction, it is necessary to 

 fix the muscle as the contraction wave crosses it, in order to study 



