Lee 
be about 7 of the maximum thickness of the body. For a 
time the right optic nerve lies directly under the left, and 
both immediately under the right bulbus olfactorius. The 
right passes straight out to its eye, but the left curves 
over towards the left side. Both reach the eye at about 
the same level, perforate the sclerotic and retina, and 
spread over the concave surface of the latter in the usual 
way. 
Owing to the fact that the left eye is situated over the 
right, the optic chiasma is less emphasised in the Plaice 
than in a symmetrical fish. 
We now proceed to the description of the eye muscle 
nerves (fig. 23), taking them in their numerical order. 
Nervus Oculomotorius—lIll. 
The nucleus of the third nerve (iii.) lies dorsally on 
the floor of the mesocoele very near the middle line, and 
mostly just over the fasciculus longitudinalis dorsalis. 
There is no crossing as in the case of the patheticus. The 
fibres of the right oculomotor curve round the fasciculus 
and pass backwards and downwards through the brain 
substance, to emerge as a large nerve on the ventral 
surface of the brain just above the lobi inferiores. Imme- 
diately it leaves the brain the nerve takes a sharp turn 
forwards, and in due course fuses with the patheticus. It 
passes downwards on the outer side of the lobus inferioris 
and between this and the v.-vii. complex. After liberating 
the patheticus again it courses downwards inside the skull, 
passes through the meninges, and enters the cup-shaped 
cavity formed by the parasphenoid and known as the eye 
muscle canal. Here it divides into a smaller upper and a 
larger lower nerve. Now the oculomotor consists mostly 
of large and well myelinated fibres, but it also contains 
