182 



PHYSIOLOGY 



ever, we are here at the limits of the demonstrably visible. It becomes 

 difficult to determine how far the appearances observed under the microscope 

 are due to actual structural differences or are produced by the unequal 

 diffraction of light by the various elements of the muscle fibre. All 

 observers are agreed that the essential contractile element is the row of 

 sarcous elements forming the muscle fibril or sarcostyle. Schafer, working 

 on the highly differentiated wing-muscle of the wasp, concludes that each 

 sarcostyle is divided by Krause's membranes (the lines in the middle of 

 each light stripe) into sarcomeres. Each sarcomere contains a darker sub- 

 stance near the centre divided into two parts by Hensen's disc. At each 

 end of the sarcomere the contents are clear and hyaline. In the act of 

 contraction, the clear material flows, according to Schafer, into tubular 

 pores in the central dark material. 



Most histologists agree in assigning to the middle part of the sarcous 

 element (the sarcomere) a denser structure than to the two ends. According 



to Macdougall, however, the lighter 

 appearance at each end of the sarcomere 

 is an optical illusion. He regards the 

 sarcous element as a cylindrical bag 

 with homogeneous contents, crossed only 

 by one or three delicate transverse 

 membranes. Krause's membrane would 



b e rigid, while the lateral wall of the 



. ... . 



sarcous element is extensible, and is 



SB, folded longitudinally, so that it can 

 bulge out and produce a shortening and 

 thickening of the whole sarcous element 



if by any means the pressure be raised in its interior. In favour of a 

 differentiation within the sarcomere itself is the fact that under certain 

 conditions it is possible to produce a precipitate, limited only to central part, 

 i.e. to the sarcous element to which Schafer assigns a tubular structure. 



When a muscle fibre, killed by osmic acid or alcohol, is examined under the micro- 

 scope by polarised light, it is seen to be made up of alternate bands of singly and doubly 

 refracting material. The doubly refracting (anisotropous) substance corresponds to 

 the dark band, and the singly refracting (isotropous) to the light band. If the living 

 fibre be examined in the same way, it is found that nearly the whole of it is doubly 

 refracting, the singly refracting substance appearing only as a meshwork with long 

 parallel meshes corresponding to the muscle prisms. In short, in a living fibre the 

 muscle prisms are anisotropous, the sarcoplasm isotropous. 



When a muscle fibre contracts, there is an apparent reversal of the situations of the 

 light and dark stripes, owing to the fact that the interstitial sarcoplasm is squeezed 

 out from between the bulging sarcomeres, and accumulates on each side of the mem- 

 branes of Krause. The accumulation of sarcoplasm in this situation makes the pre- 

 viously light striae appear dark, and the dark striae by contrast lighter than they were 

 before. That there is no true reversal of the striae is shown by examining the muscle 

 by polarised light, the two substances, isotropous and anisotropous, retaining their 

 relative positions. 



Every skeletal muscle is connected with the central nervous system 

 by nerve fibres, some conveying impressions from the muscle to the centre, 



FIG. 39. Diagram of a sarcomere in 

 a moderately extended condition, 

 A, and in a contracted condition, B ; 

 K, K, membranes of Krausc; H, 

 line or plane of Hensen ; 

 poriferous sarcous element. 



(SCHAFER.) 



