26o ZOOTOMY. 



359- The foramen of Monro {f.m\ a small aperture in 

 the inner wall of the lateral ventricle at the level of the 

 anterior end of the thalamencephalon : it communicates with 

 the third ventricle and transmits a vascular fold of pia 

 mater, the choroid plexus, which is continuous with 

 the velum interpositum. 



360. The anterior commissure {a.c), a white, trans- 

 verse cord connecting the corpora striata with one another, 

 and lying just below the foramina of Monro. 



361. The posterior commissure {p.c), a similar cord 

 uniting the thalami immediately in front of the optic 

 commissure. 



362. The fourth ventricle (7'.4), a wide flat cavity on 

 the dorsal surface of the medulla oblongata, covered, in the 

 entire brain, partly by the cerebellum, partly by pia mater. 



LVII. Carefully remove the optic commissure and the 

 dorsal wall of one or both optic lobes : observe : 



363. The aqueduct of Sylvius (Fig. 59, B), a 

 median passage placing the third and fourth ventricles in 

 communication with one another. 



364. Tlie optic ventricle (Fig. 59, A, o.v), a cavity in 

 each optic lobe, communicating internally 'ivith the aqueduct 

 of Sylvius, and with it representing the simple mesocoele of 

 the lower Vertebrata (see § 181, p. 73, and § 86, p. 23). 



LVIII. Make a longitudinal vertical section of another 

 hardened brain, and note : — 



365. The continuity of the third ventricle, aqueduct of 

 Sylvius, and fourth ventricle (Fig. 59, B, v.-^, v.\). 



366. The relation of the foramen of Monro (/.w) to the 

 third ventricle, and of the apertures of the optic ventricle to 

 the aqueduct of Sylvius. 



367. The downward continuation of the third ventricle 

 into the infundibulum {inf). 



