THE COMPOUND ETE OF INSECTS AND CRUSTACEA. 
23 
that light passing through each corneule falls upon this point 
and penetrates the clear refractive substance of the bacillum. 
The pigment surrounding the bacillum (or rather its sheath) 
isolates it complete!}^, and assists the internal refractions going 
on in the substance of the bacillum by reflecting the rays back 
on the nerve. 
The Bacillar stratum . — We now approach the most obscure 
point in the anatomy of the eye, and one which is most liable to 
misinterpretation, as it has proved also most fruitful of contro- 
versy. First in order, we may take the structure of the bacillar 
stratum as represented in our sketch of this apparatus in the 
dragon-fly. Looking at the section (fig. 6), we see stretched 
between the cornea and the optic ganglion a series of lines which 
represent the membranous sheaths of a number of bacilli, some 
of which are shown empty, whilst in others the nerve rod is 
figured within. The sheath is formed of clear membrane, but 
pigment strongly adheres to its outer surface. On its inner sur- 
face may sometimes be seen one or more small nuclei, and histo- 
logically the sheath membrane may be considered homologous 
with the connective tissue ” septa found in the retina of verte- 
brate eyes. At its outer extremity it is continuous with the 
stroma of pigment cells which lines the posterior surface of the 
cornea and is firmly attached to the corneal substance. A clear 
view of this connection can only be obtained by removing the 
pigment with the aid of solution of potash. In fig. 6 this con- 
nection is, however, well seen at the thin end of the section. Its 
inner extremity is in like manner continuous with the pigment- 
encrusted stroma of the choroid coat which covers the periphery 
of the optic ganglion. Thus then a framework of tubes fills the 
whole space between the cornea and optic ganglion; and the nerve 
fibres which spring from the optic ganglion, after piercing the 
pigment layer of the choroid, enter at the bottom of the tubes, 
and are continued forwards inside the tube (or sheath) to the 
cornea. But these nerve fibres are not like ordinary nerves. 
Their substance undergoes a remarkable metamorphosis, and 
their form an equally remarkable change. The nerve matter 
becomes highly refractive and crystalline in appearance, and the 
form of the nerve varies greatly in different eyes. Sometimes 
it swells into a club-shaped mass with ridges on four lines of its 
outer surface as soon as it enters its sheath : then in the middle 
of its course it runs to a fine thread, still preserving its quadran- 
gular shape (best seen in section) : again, as it approaches the 
cornea, it swells a second time into an oval or pear-shaped mass 
which entirely loses the character of nerve substance. In other 
instances the nerve has no bulbous swelling below, but as it ap- 
proaches the cornea swells into a four-lobed mass situate at its 
outer end. Some of the most characteristic forms are given in 
