1^2 JoiiUNAL Ol'- CoMl'AKATIVK N EUKOI-()(i Y. 



groove behind the optic lobes, following the usual course 

 thereafter. 



The optic tracts may be traced caudo-dorsad along the 

 thalamus to the mesencephalon, where they divide into a 

 median and a lateral portion. These are doubtless continu- 

 ous superficially, but are separated in the plane of the section 

 by the corpus opticum, as in birds. 



The posterior co7nmissurc occupies the usual position. It 

 is over-arched by the cephalad projection of the optic lobes 

 (see Plate XIII, Fig. 5) in which the Sylvian commissure 

 occupies the usual position. The relations are somewhal ob- 

 scured in Scaphirhynchus. 



The lobi optici are very large and resemble those of rep- 

 tiles in nearly every respect. Very large ventricles occupy 

 their centres, and where these are confluent (Fig. 9) there is 

 a curious pendant body on either side of the median line due 

 to a lateral diversion of the median walls of the tecta, below 

 their union with each other. This body, originally called 

 fornix by Carus, may perhaps retain the name suggested by 

 Fritsch, torus longitudinalis ^ since it is free from the false 

 homology involved in the other. The structure of this body 

 is very simple, consisting of dense clusters of small cells like 

 Deiter's corpuscles. 



The surface of the tectum ( i ) is clothed with a very 

 thick neuroglia layer, beneath which (2) is a rather narrow 

 uniform band of the superficial optic fibres. Within this (3) 

 is a broad band, containing peripherally a few radially 

 arranged elongate pyramidal cells. Qiiite near the ventricle 

 (4) is a multiple or single series of small pale fusiform cells, 

 with circular nuclei and interspersed Deiter's cells. The 

 ventricle is lined with the usual epithelium, (5) the fibres 

 from which radiate to the surface of the tectum, bearing, as 

 in reptiles, distinct inoblastic nuclei. The above are the 

 primary divisions of the tectum, but are susceptible of 

 farther subdivision on the basis of the course pursued by the 

 nerve fibres in each. Just ectad to the zone of small cells 



