182 



PHYSIOLOGY 



S.E, 



FIG. 39. Diagram of a sarcomere in 

 a moderately extended condition, 

 A, and in a contracted condition, B ; 

 K, K, membranes of Krause : H, 

 . line or plane of Hensen ; SE, 

 poriferous sarcous elements. 



(SCHAFER.) 



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 



A B 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 substance 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 clement (the sarco- 

 mere) a denser structure than to the two ends. 



According to Macdougall, however, the lighter appearance at each end of the sarco- 

 mere 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 be rigid, while the lateral wall of 

 the sarcous element is extensible, and is folded longi- 

 tudinally, 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 microscope by pol- 

 arised light, it is seen to be made -tip of alternate 

 bands *of singly and doubly refracting material. 

 The doubly refracting (anisotrojwus) substance 

 corresponds to the dark band, and the singly re- 

 fracting (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 re- 

 fracting, 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 ap- 

 parent 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 membranes of Krause. The accumulation of sarcoplasm in this situation 

 makes the previously light striae appear dark, and the dark stri by contrast lighter 



Fia. 40. 



ilr " ' 



Motor end -organ of a 

 lizard, gold preparation. (KiJHNE.) 

 n, nerve fibre dividing as it ap- 

 proaches the end-organ ; r, ramifi- 

 cation of axis cylinder upon 6, gran- 

 ular bed or sole of the end-organ ; 

 m, clear substance surrounding the 

 ramifications of the_axis cylinder. 



