88 



The microscopicil anatomy of the raiibraia is really quite 

 coTiplicatei, owin^ to the diversity of the tenninations and 

 connections which are established here. The base is a cro/»d- 

 ed hi'§h<«ay between the different parts of the sncephalon. The 

 fibres of the optic nerve have their termination in the dorsal 

 midbrain, the tectam -nesencephali, with a nexus of intrinsic 

 neurones and fibres of more distant origin associated with the' 

 Finally, the aqueduct of Sylvias is bordered by nervous matter 

 which is phylogenetically distinct from either the base or the 

 tectam, and this has undergone special development at certain 

 points as the nuclei of important motor neurones. 



1. The Teotum Vesenoephal i . 



The tectum mesencephali lies as an investing cap upon the 

 central gray matter which surrounds the aqueduct of Sylvius, 

 Its situation here is fraught with significance, for it repre- 

 sents an addition to the more ancient nervous structires made 

 necessary by the development of lateral eyes in the vertebrate 

 phylum. The tectum embraces the central expansions and asso- 

 ciated connections of the nerve-fibres having their origin as 

 axones of the retinal neurones. Certain neurones of the tec- 

 tum may, also, send their axones outward into the optic nerve. 

 The whole complex is, in fi-ie, the primitive visual centre. 



The tectum of Mustelus does not approach the extreme de- 

 gree of differentiation which ftamo'n y Cajal ('8?), '91) has de- 



