I.] THE FROG. 10 1 



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 

 v body." 



y. Its floor; for the most part thin prolonged 

 downwards and backwards as the infundibu- 

 him (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. 



b. 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. 



(3. 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. 



