THE LEG-MUSCLES OF THE WATER-BEETLE. 
441 
A view of muscular structure still more complicated has very lately been set forth 
by Engelmann in his exceedingly elaborate article on the subject. Engelmann enume- 
rates no less than ten distinct layers as included in each muscle-segment or compart- 
ment ( Muskelfacli of Krause). Each of the segments, according to him, is bounded at 
each end by a thin but well-marked layer ( Zwischenscheibe ), often appearing granular, 
and which is common to two contiguous segments. This corresponds with the trans- 
verse partition of Flogel. Passing inwards from these we come first upon a clear layer, 
then a somewhat dark layer ( Nehenscheibe ), which often appears to consist of a row of 
granules (the granule-layer of Flogel), then upon another clear layer, and finally, in the 
centre of the muscle-compartment, a broad dim layer ( Querscheibe ), the middle portion 
of which appears somewhat less dim than the rest (Mittelscheibe). Each of these layers 
is described with great minuteness by Engelmann. He does not, however, maintain 
that they are all to be made out in every case, but only in those instances in which a 
considerable interval separates the stripes. He further asserts that the transverse mark- 
ings are the only ones which are met with in the living fibre in its normal condition ; 
and in proof of this he states that the transverse disks appear completely homogeneous 
when viewed in situ in the living animal (as, for instance, in minute transparent Crus- 
tacea), exhibiting, therefore, no trace of longitudinal striation. The pallisade-like 
arrangement of the elements which has been described is, he maintains, due to a change 
which occurs at the moment of death of the fibre, those portions of a fibre in which 
individual rods have become visible being, according to him, no longer irritable. To 
explain these so frequent appearances, Engelmann assumes the muscular substance to 
be wholly composed of closely compressed, elongate, prismatic elements extending the 
whole length of the fibre, and exhibiting at regular intervals differences in chemical 
and physical properties, producing thus the several transverse layers. He conceives that 
at the moment of death of the fibre a coagulation and shrinking of these elements take 
place, accompanied by expression of fluid ; and in this way accounts for the fibrillar and 
granular character which the transverse disks commonly exhibit. 
Of the accounts given by the various authors with regard to the behaviour of muscle 
under polarized light, I have thought it best to say nothing in this place — partly on 
account of the irreconcilable differences of opinion on the subject, partly because the 
question has already been very fully discussed in a preceding page. 
It only remains for me briefly to point out the parts which, in the several accounts 
here quoted, seem to correspond with those enumerated by me in describing the appear- 
ance of the living muscle of Dytiscus. But, first, I would take the opportunity of 
expressing my unqualified dissent from the positive assertions of Engelmann, above 
noticed, with regard to the homogeneous appearance of muscular fibre in the living 
state, at all events so far as Dytiscus- muscle is concerned. I would affirm, on the con- 
trary, that however quickly the preparation is made (and the quicker the better), we 
find numerous muscular fibres exhibiting the structure I have described, in which the 
muscle-rods appear with perfect distinctness throughout the whole extent of the fibre, 
