THE COMPOUND EYE OF INSECTS AND CEUSTACEA. 
25 
which are studded with pigment : at the top of the nerve is the 
usual four-lobed terminal mass. In fig. 17 (Syrphus) the nerve 
is of nearly equal thickness in its whole extent up to the point 
where its ridged edges swell into a four-lobed knot embracing a 
trumpet-mouth shaped crystalline body at the top. Between 
the bacilli large tracheal tubes running from the optic ganglion 
up to the cornea are noticeable. 
Now, as may be expected, the interpretation of this curious 
structure has been variously given, and is still discussed ; for 
on the settlement of this question the explanation of vision in 
the compound eye rests. It is to be noted in the first place 
that the upper crystal-like oval or pyriform body was known 
and figured long before the peculiarities of form which the 
lower part exhibits were known. Whatever form the lower 
part takes, the crystal-like expansion above never fails ; and, 
from its delicate transparency and semi-fluidity of substance, as 
well as on account of its position close behind the cornea, it 
was never till recently suspected to be continuous with the 
nerve rod, or considered to be nervous matter. It was, there- 
fore, explained as an independent element, §.nd, in virtue of its 
position, shape, and refractive property, w^as supposed to be a 
lens and its function analogous with that of the vertebrate 
crystalline lens or vitreous humor. Thus, J. Muller, who 
first demonstrated its constant presence in all insect eyes, but 
who at the same time was unaware of the equally constant lens 
form of the inner corneal facets, looked upon this crystalline 
lens ” as the analogue of that of the vertebrate eye, and assigned 
as its office the transmission of the central ray of light penetrating 
through a corneal facet to the nerve behind it. Eudolph 
Wagner took a different view of the matter. Observing that 
this crystal-like body was composed of matter of different 
density — namely, an inner central portion more solid and refrac- 
tive, and an outer casing of softer matter — and believing that 
he had traced the nerve fibre behind it into the outer casing, 
he conceived that the inner central substance was a vitreous 
humor,” and the outer casing an expansion of nerve substance 
round it, just as the vitreous humor of the vertebrate eye is 
covered by the retinal expansion of nerve fibres. Accordingly, 
he interpreted the functions of the crystal-like body to be that 
of receiving rays of light from the cornea and forming an image 
of external objects upon its peripheral surface; that is, in direct 
contact with the retinal expansion of the nerve fibre. Wagner’s 
interpretation was obviously based on analogy of the vertebrate 
eye, but his anatomical research led him also to the discovery 
that a part at least of this crystal -like body was nerve sub- 
stance, a step in advance as compared with J. Muller’s views. 
The latter physiologist had indeed observed that the nerve 
