178 Mammalia, 



immediately behind the third ventricle and posterior com- 

 mtissure, beneath the posterior border of the Corpus Callosum 

 and above the ' iter a tertio ad quartum ventriculum/ are the 

 Corpora Quadrigemina. Each pair will be seen to be situated 

 on the cerebral aspect of the peduncle of the Cerebrum of the 

 same side : the anterior eminences {nates) far exceeding the 

 posterior {testes) in size ; and from each a lateral white band 

 {hrachia) proceeding beneath the Corpus Geniculatum internum 

 to join the Optic tract. These bodies are also connected with 

 the Cerebellum by means of a large white cord on each side, 

 the Processus a Cerebello ad Testes, or superior peduncles of the 

 Cerebellum : the thin white layer between these peduncles, 

 the Valve of Vieussens, covers in the canal leading from the 

 third to the fourth Ventricle, and gives origin to the fourth 

 nerve. 



To Expose the Iter a Tertio ad Quartum Ventricu- 

 lum, Fourth Ventricle, Arbor Vitje of Cerebellum, &c. 

 The student should now make a vertical median incision 

 through the whole length of the Brain, down to, but not quite 

 dividing, the base. 



By this means he will obtain a longitudinal section disclos- 

 ing the following points from the front : the thickened genu 

 and rostrum of the Corpus Callosum ; the anterior extremity 

 of the Fornix and the anterior Cerebral Commissure (very 

 small) in close relation ; the crescent of the third Ventricle, 

 the anterior arm descending to the infundibulum, the bend 

 passing beneath the middle or soft commissure (very large), 

 the posterior arm ascending to the Pineal Gland ; the floor 

 of the third ventricle formed by the Optic Commissure, In- 

 fundibulum, and Corpus Albicans : from the middle of the as- 

 cending arm and proceeding backwards above the Crus Cere- 

 bri, and just beneath the posterior Commissure (which is itself 

 just beneath the Pineal Gland) and the Corpora Quadrigemina 

 is the 'Iter': further back, the 'Iter' is seen floored by the 



