390 THE BRAIN 



applied to the side of the peduncle. The two optic tracts 

 converge as they pass forwards, and, finally, they are joined 

 together by a short, transverse commissural portion termed 

 the optic chiasma. The optic chiasma lies at the anterior end 

 of the interpeduncular fossa, and below the posterior end of 

 that portion of the longitudinal fissure which intervenes between 

 the inferior surfaces of the frontal lobes of the brain. The 

 optic nerves enter the antero-lateral angles of the chiasma. 



Fossa Interpeduncularis. The interpeduncular fossa is 

 the rhomboidal region which is bounded posteriorly by the 

 pons, postero-laterally by the cerebral peduncles, antero- 

 laterally by the optic tracts, and anteriorly by the optic 

 chiasma ; within the limits of the fossa the following parts 

 are situated (i) the oculo-motor nerves; (2) the substantia 

 perforata posterior ; (3) the corpora mamillaria ; and (4) 

 the tuber cinereum, with the infundibulum. 

 . Nervus Oculomotorius. Each oculo-motor nerve issues 

 from the medial side of the corresponding cerebral peduncle, 

 below the posterior perforated substance (Fig. 149). 



Substantia Perforata Posterior (O.T. Posterior Perforated 

 Space). The posterior perforated substance forms the roof or 

 superior wall of the posterior and deepest part of the inter- 

 peduncular fossa. It is a layer of grey matter in which there 

 are numerous small apertures. The apertures are caused by 

 the postero-medial central branches of the posterior cerebral 

 arteries, which were withdrawn from the apertures when the 

 pia mater was removed. 



Corpora Mamillaria. The mamillary bodies are two small, 

 white, pea-shaped eminences, placed side by side immediately 

 anterior to the posterior perforated substance. They form 

 part of the hypothalamic region, and, at a later stage of the 

 dissection, their connections with the columns of the fornix 

 will be displayed. 



Tuber Cinereum. The tuber cinereum is a slightly raised 

 field of grey matter which occupies the interval between the 

 optic chiasma, anteriorly, the corpora mamillaria, posteriorly, 

 and the optic tracts laterally. Springing from the anterior 

 part of the tuber cinereum, immediately posterior to the optic 

 chiasma, is the infundibulum or stalk of the hypophysis. When 

 the brain was removed the connection of the infundibulum 

 with the hypophysis was severed (p. 106). 



Substantise Perforate Anteriores. The anterior perfor- 



