THE LEG-MUSCLES OF THE WATER-BEETLE. 
437 
advocated, in support of the accuracy of the description I have given as to the whole of 
the muscular fibre in a state of rest appearing doubly refracting. Brucke states, in effect, 
that, in the muscles of water-beetles in the living contractile state, he has but rarely 
been able to convince himself of the existence of an isotropous substance, that it 
never is so clearly seen as in dead muscle, but even here may be missed altogether. 
Very often so little isotropous substance appears as to be only represented in the resting 
muscle by a row of points, which form coherent stripes only during contraction. 
Brucke remarks that this condition is probably the normal one during life, since he has 
most frequently come across it in perfectly fresh living muscle*. It is not difficult to 
identify Brucke’s isotropous substance, which he also terms Zwischensubstanz , with 
the heads of the isotropous muscle-rods above described. But it may be asked, How 
comes it that in dead muscular fibres, especially if they have been placed in alcohol and 
subsequently mounted in Canada balsam or dammar-varnish, even though apparently 
fully extended, we so often observe broad alternating stripes of anisotropous and 
isotropous substance \ To this I would answer that, although these fibres are not 
actually shortened, yet the elements composing them have tended to assume the con- 
dition that ordinarily accompanies contraction of the fibre (see below). And this 
opinion is fully confirmed by Brucke’s statementf, that the isotropous disks are always 
broadest in such fibres as have been prevented from shortening at the time of death. 
It is, in fact, as if we took a fibre that had become fixed at death in the contracted 
state and mechanically stretched it out to its original length ; in which case we may 
readily conceive the anisotropous and isotropous disks which are present in that state of 
the fibre to be equally stretched out. 
And here I am tempted to offer a conjecture respecting the probable changes which 
take place during the contraction of muscle in the disposition of the structures which I 
have endeavoured to describe. 
It is conceivable that the anisotropous ground-substance is the true contractile part of 
the fibre, and that it is allied in nature to ordinary protoplasm, but without the 
granular character commonly met with in the latter J. The muscle-rods may be 
imagined to be composed of a labile, and at the same time elastic substance of semifluid 
consistence, possessed of considerable refracting power on light, and in all probability 
devoid of vital contractility. 
* “ Untersuchungen iiber den Bau der Muskelfasern mit Hiilfe des polarisirten Lichtes,” Wiener Denk- 
schriften, xv. p. 79. 
t Loc. cit. p. 80. 
+ Since the above was written my attention has been drawn to a paper by M. Schultze (Reich, und Dr 
Bois-Rethond’s Archiv, 1861), in which that author gives it as his opinion that the substance between the 
rod-shaped sarcous elements is the remains of the unaltered protoplasm of the embryonal muscle-cell. He 
would appear, however, to ascribe to it rather a nutrient than a contractile function, still looking upon these 
as the active elements of the fibre. 
3 N 
MDCCCLXXIII. 
