1886.] Central Nervous System of Plugio*tomata. 



13 



fibres are not present in the Plagiostomata, the fibres of the ventral 

 longitudinal columns varying very slightly in size ; but perhaps, it 

 may be mentioned here that large fibres, two in number, occupying 

 positions corresponding to those of the Mauthner's fibres in Teleostei, 

 occur in Ceratodus ; they have the peculiarity of possessing several 

 axis cylinders inclosed in a single medullary sheath. 



The ventral columns form projecting longitudinal cords in the floor 

 of the fourth ventricle. They can be traced into the ventral side of 

 the posterior commissure which occupies the usual place at the pos- 

 terior boundary of the third ventricle. 



The lateral columns on passing forward diminish greatly in number, 

 the internal fibres are lost in the neighbourhood of the posterior 

 commissure ; those that are external seem to join the transverse fibres 

 of the optic lobe, those between the two disappear in the region above 

 the hypoarium, some crossing the crura cerebri which disappear in the 

 same region. v . 



The optic nerves form a chiasma, the lower part of which is formed 

 by the nerves of the two sides intersecting each other in bundles, but 

 in the upper part the remainder cross each other en masse. The 

 principal origin of this nerve is the optic lobe, where the outer two- 

 thirds are occupied by its tract ; a few fibres, however, are derived 

 from the hypoarium. 



The oculo motorii are derived from two ganglia situated on the 

 floor of the aqueduct of Sylvius ; they pass nearly straight down to 

 the ventral surface of the medulla oblongata. At this region there is 

 a system of transverse commissures connected wdth the second layer 

 of the optic lobe, which corresponds to the commissura ansulata of 

 Teleostei (Gottsche).* 



The ganglion of origin of the trochlearis was not found, but the 

 fibres decussate at a part between the optic lobe and the cerebellum 

 corresponding to the valve of Vieussens. 



The trifacial is derived from three roots, one of which comes for- 

 ward from the posterior part of the medulla oblongata, where it can 

 be traced into the lateral columns. The other comes backward 

 through the tuberosity of the trifacial by the side of the medulla ; 

 these two cross each other at their entrance into the nerve, the third 

 comes from a group of cells in the grey substance of the floor of the 

 fourth ventricle. 



The abduceus can be traced from the ventral surface of the medulla 

 oblongata at about its centre into the lower edge of the ventral 

 longitudinal columns. 



The facial can be traced into a small bundle of fibres which passes 

 backward into the spinal cord in the substantia gelatinosa centralis 

 just above the central canal. 



* Loc. cit., p. 439. 



