THE LEG-MUSCLES OF THE "WATER-BEETLE. 
431 
of the bright stripe, having become merged into a single dark transverse streak, having 
a granular look. 
If we imagine this process to go on still further — that is to say, if the shafts go on 
increasing in size at the expense of the rod-heads until each muscle-rod is of uniform 
diameter throughout, the ends of the rods of different series being in close apposition, 
it may well be imagined (the smallness of the objects with which we have to deal being 
taken into consideration) that it will be extremely difficult to detect the points of junc- 
tion between the rods of successive series. In this way the continuous longitudinal 
fibrillation which is often seen in the muscles of the Water-beetle may be accounted 
for. That each fibril, however, is not in reality continuous throughout the fibre, but 
consists of a succession of shorter units, the ends of which are in close apposition, is 
shown, not only by the phenomena which occur when such a muscular fibre is thrown 
into contraction, and which will be referred to further on, but also by the appearances 
presented when a portion of the fibre comes under observation in which these units are 
thrown into zigzag instead of lying in the same straight line, in which case the angles 
of the zigzag represent the points of junction of the muscle-rods. 
It is worthy of note that in all fibres like that we are now considering (that is to say, 
all in which the rod-heads are of the same diameter as the shaft, so as no longer to 
appear as distinct objects), the bright transverse bands have entirely disappeared, and 
the substance between the rods is of a uniform appearance throughout. 
II. Appearance of Contracted Muscle. 
In contracted muscle (fig. 2, C) we observe, besides the general increase in thickness 
of the fibre and the approximation of the cross stria?, that the latter have become com- 
pletely altered in character. Dark and light transverse stripes now appear, which are 
of nearly equal size : the former, instead of being merely dim as in the resting muscle, 
have a decidedly dark aspect ; the light stripes, on the other hand, present a very bright 
appearance, without any indication of a row of dots, but with sometimes a faint longi- 
tudinal streaking. 
In order to explain this difference between a fibre at rest and the same fibre in the 
contracted state, it is necessary to follow all the stages of the process. Owing to the 
extreme suddenness and rapidity with which the contraction supervenes, and the 
amount of displacement which immediately precedes and accompanies it, it becomes a 
matter of exceeding difficulty to follow the details of the process with the high powers 
which it is necessary to employ. However, it has occasionally happened to me to 
observe a contraction proceeding at a very much less rapid rate, and in such cases it 
was possible to make out the following changes : — 
The first thing seen is that the bright transverse bands become approximated 
throughout the contracting portion (P), and with them the dark transverse lines in 
their middle, which, as previously shown, are made up of a double row of ; dets (rod- 
heads). As the contraction continues the rod-heads become larger, and by this means 
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