On the Relation of the isotropous to the anisotropous Layers in striped Muscles. 539 



Here we find the total length of the contracted "element" is either 

 the same as (case X) or but very slightly shorter than the anisotropous 

 layer in the extended "element". In those cases in which it is 

 shorter, the diminution amounts to V?"* i^ case I, Vb"" ^^ case II, 

 V5"' in case XI, giving an average (including case X) of ca. Vg '**• 

 This amount of contraction is not too large to be attributed to 

 unavoidable error in making these very difficult and delicate meas- 

 urements. In support of this it is specially noteworthy that case 

 X shows that the contracted "element" is exactly the length of the 

 anisotropous layer in the expanded element, and that case VIII 

 implies nearly the same thing. In this latter case we have the 

 commencement of the fully contracted stage , with the contracted 

 „element" still longer than the anisotropous layer in the expanded 

 „element". 



This Vg or, taking the length of the anisotropous layer as, on the 

 average, half of the whole expanded "element". Vie of the latter, is 

 hardly worth considering, when we remember that the "element" is often 

 contracted 50% and more. As the isotropous layers are often more 

 than half of the whole length of the "element", their absorption in the 

 anisotropous layer alone is enough to explain the strongest contraction. 



Fig. 24 embodies these measurements in a diagram, which it 

 seems to me shows conclusively that the essential element in con- 

 traction is the absorption of the isotropous by the anisotropous sub- 

 stance. This diagram also shows at a glance that if we grant Engel- 

 mann's persistent isotropous layer in the contracted "element" (i") the 

 shortening of a to a" is a very small factor in the contraction of the 

 „element" ; this is also clear from the cases themselves, where the iso- 

 tropous layer is always seen to have shortened far more than the 

 anisotropous. 



Beyond this testimony from Engelmann's tables, we may mention 

 further that Krause, one of the founders of our knowledge of striped 

 muscle, maintained that the anisotropous layers suffered no con- 

 traction. 



On turning to the wing muscles of Flies, we have different con- 

 ditions. Schäfer describes the anisotropous layers as contracting 

 longitudinally when laterally swelled up by the absorption of the iso- 

 tropous substance. It seems to me that the flowing in and out of the 

 isotropous layers into the anisotropous pits would bring about all the 

 contraction required of these muscles, which, from the nature of the 



