212 E. A. Schäfer, 



traction of the musle-column in white of egg, and other considerations, 

 render extremely improbable), to explain the increase of the clear 

 interval, when the muscle is stretched, is to suppose that the semi-fluid 

 substance which, we assume, occupies it, has appeared at the expense of 

 the sarcous element, has, in fact by virtue of the stretching of the 

 fibre become expressed out of the sarcous element as fluid would under 

 like circumstances be expressed from the pores of a sponge, supposing 

 this to be contained in an elastic tube which was put upon the stretch. 

 To carry the simile further and to apply it to the converse conditions 

 which accompany the passage from extension to contraction, it is clear 

 that when the stretching force is relaxed the sudden re-imbibition of 

 the fluid by capillarity into the pores of the sponge will produce a 

 sudden shortening and bulging of the segment of the tube containing 

 it. x4nd although there would of course, be no actual production of 

 force, nevertheless the force of the retraction thus effected by capillarity 

 and elasticity combined might, if concentrated within a short period 

 of time, produce during that time a relatively considerable effect. It 

 has been held by several previous writers upon the subject, that during 

 the contraction of a fibre there is a passage of fluid from the clear 

 intervals into the sarcous elements, and back again into the clear 

 intervals during its relaxation. Tins shows that the interchange of 

 fluid between the clear intervals and the sarcous elements is a fact 

 which has been recognised by other observers. Merkel speaks of a 

 passage of part of the substance of the sarcous element into the clear 

 interval during contraction, and of an imbibition of the fluid of the inter- 

 val by the material of the sarcous element. Krause describes the sar- 

 cous element as formed of minute rod-like fibrils and supposes that 

 in contraction the fluid of the clear spaces passes between these 

 fibrils, and thus increases the bulk of the sarcous element at the expense 

 of the clear interval. No one, however, so far as I am aware, has 

 attempted to explain the mechanism of such interchange. And without 

 some reasonable explanation of this, mere theorizing does not carry 

 our conception of the process of contraction any further. 



In order to obtain such explanation, we must go back to the 

 consideration of the minute structure of the sarcous elements of the wing- 



