THE LEG-MUSCLES OF THE WATEK-BEETLE. 
435 
traction, however, it seems to tend more towards the middle of the muscle-rods between 
their shafts, thus producing bulgings at the exterior of the fibre opposite these. 
VI. Appearances under Polarized Light. 
Although at first sight it may seem an easy matter to determine which parts of a 
muscular fibre are singly and which doubly refracting, yet, in tact (owing no doubt 
principally to the complex optical conditions which are produced by the heterogeneity 
of the structures through which the light has to pass), there is hardly any subject in 
histology in which greater difficulty occurs in arriving at a correct result. Accordingly 
scarcely any two observers who have carefully investigated living muscular fibre with 
this object have arrived at precisely the same conclusion. One of the chief difficulties 
is that of obtaining the muscular fibres in a sufficiently isolated state ; for the addition 
of any fluid is quite inadmissible, and without such addition isolation of the individual 
fibres is liable to be accompanied by desiccation and loss of the normal structural 
appearances. Of course the presence of other fibres under or over the particular one 
we are examining may considerably complicate our observations. 
Having fully experienced the difficulties of this part of the investigation, I have 
taken all the more pains to arrive at a trustworthy result, and therefore feel the less 
diffidence in stating my own conclusions, although they differ materially from those 
of other observers, with the partial exception perhaps of Heppner. 
The method adopted has been to make the preparation upon a cover-glass, and to 
enclose it in a moist chamber in the manner already explained. In this way all possi- 
bility of pressure is avoided, and fibres are frequently obtained partially isolated from 
the rest and perfectly living and contractile. For the purpose of illumination, the 
oxy calcium light has been employed. For observing the optical conditions, as well as 
other phenomena, of contracting muscle, I have found it a good plan to pass into the 
moist chamber (whilst a fibre is under observation) air containing a trace of alcohol 
vapour. Hartnack’s polarization-apparatus was used throughout. 
The opinion, then, that I have been led to form, by a careful comparison of the 
appearances presented by muscular fibres in the states respectively of rest and ot 
contraction, when placed between crossed Nicols, and with their axes at an angle ot 
about 45° with the principal plane of either prism, may be shortly stated as follows : — 
Leaving out of consideration the sarcolemma, nuclei, &c., the whole of the proper 
substance of a muscular fibre is anisotropous, with the exception of the structures here 
designated muscle-rods , which are isotropous. 
If we observe under the above conditions a living muscular fibre in the state of rest, 
the whole fibre appears illuminated on the dark field ; and in consequence of this 
illumination we are able to make out the structural points (such, for example, as the 
presence of muscle-rods) almost as well as when it is illuminated by ordinary light. 
The rods are clearly distinguishable as dim streaks on the bright ground of the fibre, 
the heads .giving the appearance, by their contiguity, of a single or double dark dotted 
