THE EYE AND OPTIC TRACT OF INSECTS. 
217 
the ommateum or ommatidium as I shall call it, adopting the 
term introduced by Carriere (3) there is situated in the region 
of the base of the pseudocone, a large pigment-cell (figs. 3 and 
10, pg..^). Each of these pigment- cells consists of a central 
rounded portion containing a large spherical nucleus and two 
or more delicate processes which pass externally {ex.) to the 
flat portions of the internal surface of the cornea, and internally 
{in.) to accompany the retinulse towards the basilar membrane. 
The rhabdom of Musca consists of a bundle of six long 
delicate chitinous rods, more or less firmly united together 
(fig. 3, rh.). The rhabdomeres or elements of the rhabdom 
are more clearly distinguished from one another in the outer 
part of their course (fig. 4) than they are in the inner part of it 
(fig. 5.) The rhabdom is surrounded by the retinulse. These 
are six in number, and in the outer region of their course are 
free from one another (fig. 4, r.), but in the inner region are 
fused into a sheath. 
Each retinula element possesses a nucleus just behind the 
nuclei of the pseudocone (fig. 3, nr.) and some of them possess 
an additional nucleus in the middle of their course (fig. 3, nr v ). 
Thus we find a ring of six retinular nuclei around the central 
rhabdom, just behind the pseudocone and two or three nuclei 
somewhat more irregular in position about half way down. 
When quite fresh the retinulse are of a deep carmine colour, 
but this soon fades away under the influence of the light, and the 
retinulse are left with a yellowish-brown colour. Between the 
ommatidia internally there are found pigment-cells (figs. 3, 
pg. 3 ), each of which stands on the basilar membrane and sends 
a fine process outwards towards the internal process of the 
external pigment-cell {pg. 2 )> The pigment-cells are filled with 
bright carmine-coloured granules which change to a deep brown 
colour when treated with alcohol. 
The basilar membrane of Musca is very thin and perforated 
for the passage of tracheal diverticula and the optic nerve- 
fibrils. 
Between the ommatidia are situated long tubular thin-walled 
air- sacs, which may be traced into connection with the nume- 
