438 
MU. E. A. SCHAFEU ON THE MINUTE STEUCTUUE OF 
When a portion of nmscle is called into activity, the contractile substance, or at least 
that portion of it which is between the shafts of the rods, shortens in the direction of 
the axis of the fibre, swelling out correspondingly in the lateral direction ; the rods 
consequently become compressed in the centre, so that the substance of which they 
are composed tends to accumulate towards the ends. But these become also enlarged 
in another way ; for the effect of the shortening of the fibre would be, first to press the 
heads of those in adjacent series against one another, and then, by a continuation of this 
process, to force them down upon their own shafts, encroaching upon the substance of 
these and becoming larger at their expense. The change of form may be most aptly 
compared to that which happens when the end of a thread of glass is put into a flame, 
the glass as it softens running up into a spheroid. 
When the contraction ceases all the processes are reversed : it is possible that 
elasticity of the muscle-rods is an agent in restoring the fibre to its original length. 
To recapitulate : — The general result of my investigations upon muscular fibre in the 
living state has been to induce me to regard the less strongly refracting intermediate 
substance which pervades the whole fibre as the contractile, irritable, and consequently 
essential part of the muscle, whereas the more refracting substance (which, as a rule, 
in the leg-muscles of Dytiscus appears in the form of distinct rod-shaped particles 
regularly arranged, but unconnected with one another) is, I think, to be regarded as 
performing a passive function only, and consequently as unessential to the idea of a 
muscular fibre, so far as its function of contractility is concerned. It is possible that 
this substance is analogous to that of which the granules ordinarily found in protoplasm 
are composed, being, however, in the case of striped muscle, for some reason at present 
unknown to us, arranged in a definite manner. That the ground-substance of muscle 
is doubly refracting, whereas ordinary protoplasm, such as that of which the pale cor- 
puscles of the blood are composed, is not, cannot be taken as a proof of dissimilarity in 
nature ; for, as is well known, the substance of which the cells of plain muscular fibre is 
made up (the protoplasmic nature of which few, I imagine, would be prepared to deny) 
is also doubly refracting. And since, according to the view here adopted, the more 
refracting substance is to be regarded as the non-essential part of the fibre, we must not 
be surprised if differences occur in the mode of its arrangement in the muscles of 
different animals, or even in different muscles of the same animal. In other words, it does 
not seem imperative that a typical structure should be selected to which all striped 
muscles must necessarily exactly conform; but, on the contrary, differences in the 
arrangement of the non-essential elements may, and undoubtedly do, occur without 
corresponding differences in the essential functional activity of the fibres. 
A great deal has been written within the last few years with regard to the structure 
of striped muscular fibre. Previously to 1868 the view's of Bowman, as modified by 
Rollett, were very generally accepted as affording an explanation of most of the appear- 
