117 



be about | 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 siirface of the latter in the usual 

 way. 



Owing to the fact that the left eye is situated over the 

 right, the optic ehiasma 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. 



N e r V u s c u 1 o m t r i u s — III. 



The nucleus of the third nerve (iii.) lies dorsally on 

 the fioor 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 paiasphenoid and known as the eye 

 muscle canal. Here it divides into a smaller upper and a 

 larger lower nerve. Now the oculomotoi' consists mostly 

 of large and well myelinated fibres, but it also contains 



