146 



bundles of minute tubules lying within the sarcous elements, and these, taking 

 up fluid from the clear part of the sarcous element during contraction, shorten 

 and bulge. This observation might be taken as a confirmation of Bernstein's 

 view ; nevertheless, as I shall show later, in connection with the structure and 

 action of wing muscle, it is founded on an incorrect observation, and is in part, 

 at any rate, to be quite differently interpreted. The necessity for the existence 

 of a spongy network, however, remains, and it seems to me that no better con- 

 firmation of the presence of such a structure could be desired, than the observation 

 that the middle of the sarcomere remains doubly refracting towards polarised 

 light even after the darkly-staining fluid has retreated, during contraction, from 

 that region. It is, in fact, the spongy network which is doubly refracting. Into 

 it, in the expanded state of the muscle, the greater part of the darkly-staining 

 material is drawn through the minute channels which I have described above as 

 occurring in the clear region of the sarcomere, leaving only a small quantity 

 of residual hyaloplasm behind, in the neighbourhood of Krause's membrane. 

 At contraction, the excitatory stimulus must decrease the surface tension of the 

 hyaloplasm, and the portion that has been drawn through the tubules into the 

 doubly refracting region, and which now forms the striation (movable hyalo- 

 plasm), is rapidly sucked back and accumulates at either end of the sarcomere, 

 in the region of Krause's membrane. That contraction is the result of a decrease, 

 not increase of surface tension was first pointed out in a paper by Dr. T. Brails- 

 ford Robertson (1909). 



At relaxation, on the other hand, there is an increase of tension ; the 

 hyaloplasm from either end of the sarcomere is sucked rapidly up the tubules 

 again and enters the doubly refracting spongy network. In many muscles the 

 fusion of the hyaloplasm from either end of the sarcomere is not complete, and 

 we obtain a pale line in the middle of the striation, which is Hensen's line (fig. 5, 

 pi. xi.). It is especially clearly seen in the sarcostyles of the wing muscles of 

 insects. 



''"mil 



■hi 



^j^?j® 







' i 1 iVi ? 



^ 



D 



A 1{ C 



Diagram of two sarcomeres midergoing contraction. The dark substance 

 is the hyaloplasm (movable and residual). The dotted part is the doubly- 

 refracting "spongy network." Z is Krause's membrane. 



