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 ; SB, poriferous sarcous 

 element. (SCHAFER.) 



THE STRUCTURE OF VOLUNTARY MUSCLE 203 



Most histologists agree in assigning to the middle part of the 

 sarcous element a denser structure than to the two ends. According 

 to Macdougall, however, the lighter appearance at each end of the 

 sarcous element 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 exten- 

 sible, and is folded longitudinally, 

 so that it can bulge out and 

 produce a shortening and thick- 

 ening of the whole sarcous ele- 

 ment if by any means the pres- 

 sure be raised in its interior. In 

 favour of a differentiation within 

 the sarcous element itself is the 

 fact that under certain conditions 

 it is possible to produce a preci- 

 pitate, limited only to the central part of the sarcouselement, i.e. the 

 part to which Schafer assigns a tubular structure. 



When a muscle fibre, killed by osmic acid or alcohol, is examined 

 under the microscope 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 membranes of Krause. 

 The accumulation of sarcoplasm in this situation makes the previously 

 light strise appear dark, and the dark striae by contrast lighter than 

 they were before. That there is no true reversal of the stria? 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, the others acting as the path of the motor impulses from 

 the centre to the muscle. These latter the motor nerves- end in 

 the muscular fibre itself, by means of a special end-organ the motor 



