I.] THE FROG. " lOI 



a. The median ventricles (§ iv. a) ; note their limits 

 and their relationships to the several constituents 

 of the brain. Take especial note of the third 

 ventride ; its walls are modified as follows 



a. Front wall ; greatly thickened to form the 

 Ia?7iina terminaiis (iv. a. y). 



^. Roof; thin and delicate, prolonged upwards 

 and forwards to give attachment to the "pineal 

 body." 



y. Its floor; for the most part thin prolonged 

 downwards and backwards as the infundibu- 

 luni (ii. e. /3) to meet the pituitary body ; 

 thickened at its middle around the optic 

 chiasma (ii. d). 



S. The foramen of Monro ; lying immediately 

 ^ above the lamina terminaiis and large enough 



to admit a good-sized bristle. 



b. The co7n??iissures ; small but definite tracts passing 



between the opposite halves of the brain ; rela- 

 tively whitish in colour. Look for their cut 

 edges with a powerfid hand lens^ as under — 



a. The posterior commissure ; lying in the sub- 

 stance of the roof, immediately in front of 

 the optic lobe. 



y8. The corpus callosum ; represented by a well- 

 defined tract traversing the head of the lamina 

 terminaiis, immediately below and behind 

 the foramen of Monro. 



y. The afiterior commissure ; smaller than the 

 above and running through the middle of the 

 lamina terminaiis. 



