143 



Equally contradictory views have been held on the actual process of con- 

 traction. The observation of Schafer that isolated sarcostyles of insect wing 

 muscles can contract showed clearly that it is not in the sarcoplasm that we 

 must look for the contractile mechanism, as Cajal believed. Bov/man, in his 

 original communication, had regarded contraction as taking place by the swelling 

 of the striations. McDougall believed the swelling to be due to the passage,, 

 probably by osmosis, of water from the sarcoplasm into the fibrils. Schafer 

 has combated this view, and the inherent slowness of the process of diffusion 

 under an osmotic pressure prevents its acceptance when we consider that during 

 tetanus, mammalian muscles may contract at the rate of at least fifty times per 

 second. 



But even those who consider the contractile process to be confined to the 

 fibrils differ in their interpretation of the action. According to Merkel, the dark 

 portion of each sarcomere (sarcous element) diffuses during contraction through 

 the sarcomere and accumulates around Krause's membrane, while the light por- 

 tion in turn moves towards Hensen's line, so that there occurs a reversal of 

 striation. Engelmann, on the other hand, concluded in 1873 that during con- 

 traction the dark part of each sarcomere, which was doubly refracting towards 

 polarised light, remained, even in the fully contracted state, in the middle of 

 the fibre, and that any reversal of striations in Merkel's sense was due to 

 peculiar optical effects. This view, mainly through the work of Schafer, is 

 maintained by most competent histologists to-day. Schafer (1891) described 

 a swelling of the sarcous elements in the highly differentiated wing muscles of 

 insects, a process which he regarded as being due to the passage of the clear 

 singly refracting material from the region of Krause's membrane into the sarcous 

 element. By this means Krause's membrane became drawn up nearer to the 

 sarcous elements ; the latter bulged, due to the absorption of some of the singly 

 refracting substance, and the slight increase in volume of the doubly refracting 

 material that Engelmann had observed, was accounted for. 



Rutherford (1897) took a view intermediate between that of Merkel on 

 the one hand, and of Engelmann and Schafer on the other. He conceived con- 

 traction as consisting of two processes: (a) A flow of clear (singly refracting) 

 material into the sarcous element (similar to that of Schafer), and (b) a sub- 

 sequent accumulation of dark material in the region of Krause's membrane (as 

 observed by Merkel). Rutherford's view has not found acceptance by modern 

 histologists. 



Nevertheless, an examination of fixed contraction waves in muscle fibres 

 shows most clearly that a reversal of striations, even if only apparent, has 

 taken place. Nothing more convincing than the beautiful figure given by 

 Schafer of a contraction wave in a muscle fibre of Dytisciis could be desired. 

 Schafer accounts for the reversal in the following ingenious manner : "As the 

 sarcous elements swell out more and more during contraction, the interfibrillar 

 sarcoplasm becomes pressed from the spaces between the sarcous elements of 

 adjacent fibrils, and accumulates in the region of Krause's membrane." It is 

 this movement of the sarcoplasm which he regards as causing the reversal of 

 striations. 



The Structure^ Action^ and Development of Striated Muscle Fibres 

 (excluding Wing-Muscles of Insects). 



Methods employed. — For the examination of the minute structure of muscle 

 fibre, I have throughout employed sections cut from paraffin blocks, and stained 

 with iron haematoxylin. Cold Bouin's Piro-Formol mixture was used as a 

 fixative (hot fixative must be avoided, as it causes rupture of the sarcous 

 elements). Gold chloride preparations were also used, but, though these give 

 sharply defined preparations of entire fibres, yet for the minute details of 



