NERVOUS SYSTEM. VERTEBRATA. 



91 



ganglia and olfactory tracts. At the root of the olfactory 

 tract can be seen the cut edge of the commissura inter- 

 lobularis (fig. 25, ANT. COMM.). The common cavity of 

 the infundibulum and saccus vasculosus is clearly shown ; 

 in the side wall of the former is a minute opening that leads 

 into the ventricle of the right lobus inferior. 



At the anterior end of the tectum opticum lies the 

 section surface of the small posterior commissure. 



A narrow passage leads from the aqueduct of Sylvius 

 into the cavity of the optic lobes *. In the section this cavity 

 is largely obliterated by one of a pair of longitudinal ridges 

 (tori longitudinales) that lie on either side of the mid-dorsal 



CORP. SIR.. 

 PALL. 



Fig. 25. 



TECT. OPT. 

 TORUS. LONGIT. I 



POST COMM. \ 



AQ. SYL. 



CER. 



LOB. INF. 



OPT. H / 3* VENT. ' ^^^ INh \ ' SAC.VASC. 



ANT. COMM. FOR. LOB. INF. VALV.CER. 



Brain of Gadus morrhua in sagittal section. 



line of the tectum. The torus is thickest anteriorly where 

 it abuts on the posterior commissure, and gradually fades 

 away posteriorly. 



The hinder part of the optic ventricle is occupied by a 

 forvvardly projecting process of the cerebellum (valvula 

 cerebelli) . This structure is formed by the invagination of 

 the anterior parts of the cerebellum into the cavity of the 

 mid-brain, and thus in sagittal section shows two superposed 

 layers the lower one continuous behind with the cere- 

 bellum and passing in front by reduplication into the dorsal 

 layer. The latter is closely applied to the first, and is 

 continuous posteriorly with the hinder margin of the tectum 

 opticum. Beneath the posterior lobe of the cerebellum can 

 * In fig. 25 the leading line from AQ.SYL. points somewhat too far back. 



