I.] THE FROG. 101 



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 

 ventricle; its walls are modified as follows 



a. Front wall; greatly thickened to form the 



lamina terminalis (iv. a. y). 

 ft. 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- 



lum (ii. e. ft) to meet the pituitary body; 



thickened at its middle around the optic 



chiasma (ii. d). 

 8. The foramen of Monro ; lying immediately 



above the lamina terminalis and large enough 



to admit a good-sized bristle. 



/;. The commissures ; small but definite tracts passing 

 between the opposite halves of the brain ; rela- 

 tively whitish in colour. Look for their cut 

 edges with a powerful hand lens, as under 



a. The posterior commissure ; lying in the sub- 

 stance of the roof, immediately in front of 

 the optic lobe. 



ft. The corpus callosum ; represented by a well- 

 defined tract traversing the head of the lamina 

 terminalis, immediately below and behind 

 the foramen of Monro. 



y. The anterior commissure; smaller than the 

 above and running through the middle of the 

 lamina terminalis. 



