46 
G. H. Parker 
of almost pure fibrillär material, is a confused mass of fibrillae and 
nerve fibres. Facts concerning the courses taken by the fibres in 
this gauglion are ratber scanty. So far as my observations extend, 
all the neiirons of the fourth order terminate in this ganglion, and, 
excepting the fibres that enter the optic nerve directly from the second 
ganglion, all optic fibres likewise terminate here. The courses taken 
by the fibres are, hov^ever, not easily worked out. The fibres that 
come from the posterior end of the tbird ganglion (neurons of the 
fourth Order) make their way into a reiatively small eminence on the 
dorsal side of the fourth ganglion (PL 2 Figs. 55, 56, x], and termi- 
nate there in the characteristic fibrillation. From this eminence a new 
and peculiarly marked set of fibres (Fig. 56, n.d) proceeds proxi- 
mally through the ganglion to the dorsal part of the optic nerve, 
whence they can be traced to the brain. A second System of fibres, 
though less easily demonstrable, can be traced through the ventral 
part of the ganglion. The fibres from the anterior portion of the 
third ganglion terminate in the ventral part of the fourth ganglion, 
from which region a new set of fibres passes directly to the brain 
through the ventral part of the optic nerve. These two Systems are 
the only ones about which I can speak with confidence; the others 
that must occupy intermediate positions I have been unable to iden- 
tify with certainty. 
c. Optic Nerve. 
The optic nerve, a conspicuous structure in ordinary dissection, 
extends from the fourth optic ganglion to the optic lobe of the brain. 
Its finer structure is best understood from transverse sections (PI. 2 
Fig. 38), where it will be seen to be composed of a vast number of 
fibres, of which the dorsal ones {n.d) form a peculiar group eccen- 
trically sunounded by the others. These dorsal fibres arise, as al- 
ready mentioned, in an eminence on the dorsal face of the fourth 
ganglion and make their way through the dorsal part of the optic 
nerve to the brain. They were recognised long ago by Dietl 
(76, PI. 37 Fig. 24), and they differ from the other fibres of the 
optic nerve in that their individual limits are very poorly defined. 
This is due, I believe, to the imperfect development of their sheaths 
of Schwann. If the frequency with which the nuclei of these 
sheaths occur be taken as an indication of the degree to which the 
sheaths are developed, a striking difference in this respect will be 
noticed between the two parts of the optic nerve. Comparing the 
