116 



l)ull)iis has no direct connection with its nerve in front as 

 on the right side, nor does it extend as far forwards as the 

 right one. There is mnch more difference in the size of 

 the two olfactory nerves than one would expect from the 

 sizes of their bulbs: the left Ijcing' only j the size of the 

 right. N^or do its fibres take such an intense stain with 

 the osniic acid. For some distance the two olfactory 

 nerves course together, but finally the left separates from 

 the right and passes upwards towards the eyeless side of 

 the body. It then traverses the olfactory foramen in the 

 left prefrontal, and at once passes straight upwards to 

 break up in the olfactory laminfe of the left nasal 

 chamber. The left nasal organ is much smaller than the 

 right (cp. fig. 25, ??. olf., n. olf.^), and hence the small left 

 nerve. It is also situated somewhat behind the right, and 

 therefore the left olfactory is the shorter of the two. 



N e r V i; s p t i c u s — II. 



As in all lower vertebrates, the fibres of the optic 



nerve arise mostly from the roof of the mid-brain (tectum 



opticum), and as is usual in Teleosts they pass forwards 



over the ventricle to collect at the anterior extremity of 



the optic lobe, and then course sharply downwards and 



forwards to reach the surface of the brain. The optic 



chiasma is a simple crossing without any intermingling of 



fibres, so that the nerve to the right eye, for example, arises 



exclusively from the left side of the brain. As in Menidia 



the nerve to the left eye is uppermost at the crossing. 



Each optic nerve, as is usual in Teleosts, consists of a thin 



wide ribbon so thrown into longitudinal folds as to form 



a round nerve, and each exhibits 8^ folds.* If the optic 



nerve could be flattened out the width of the ribbon would 



* The number of the folds increases with the size of the nerve, judging 

 from our sections of young plaice at different stages, and also from the 

 condition in the adult (see fig. 28). 



