of Edinhurgli, Session 1883-84. 
291 
How is it that if we treat living protoplasm with certain reagents it 
breaks up into two substances? How are we to explain this 
observed fact ? Is there any case known to chemists or physicists 
where a chemical reagent separates a substance, at first apparently 
homogeneous, into two distinct substances which afterwards reunite ; 
or where an electrical or mechanical stimulus temporarily separates 
a complex mixture into its components ? I am not aware of any 
parallel case ; hut the absence of an explanation underlying the 
phenomenon of aggregation itself is after all entirely outside, and 
subsequent to both the main thesis and the speculative corollaries 
of the present paper. 
III. An Hypothesis of Contractility. 
If we imagine a single drop of more or less fluid substance sus- 
pended in a surrounding medium with which it does not mix, its 
surf ace- tensions tend to keep it in a spherical form. This can of 
course he observed in a drop of oil or water, most conveniently in 
the well-known experiment in which oil is suspended in a mixture 
of alcohol and water of the same specific gravity. If now we 
interfere and elongate this drop, its surface tensions at once enable 
it to resume the spherical form, even against the resistance of the 
surrounding medium. It is inevitable to transfer this simple 
physical conception to explain the function of muscle. If the 
elongated components of the muscular fibre he more or less fluid 
(as we almost certainly know them to be, whether aggregation 
granules or not), they must needs also possess surface tensions; 
they must, therefore, also tend to shorten and broaden, and draw 
themselves into a sphere. And if all these multitudinous elements 
of the muscle are shortening ^and broadening, we have of course an 
explanation of that general shortening and broadening which we 
term the contraction of a muscle. 
Moreover, just as the ^‘contracting” drop of oil overcomes a 
resistance, so the “contracting” muscle overcomes a resistance equal 
to the sum of the minute resistances which the millions or thou- 
sands of millions of simultaneously contracting elements can over- 
come. Finally, an expenditure of energy must needs take place, 
and of this the “negative variation” of contracting muscle (perhaps 
of the contracting oil-drop also ?) may afford the indication. 
