PARS OPTICA HYPOTHALAMI.
This division of the telencephalon includes : The lamina terminalis, The chiasma opticum, with the tractus optici, The tuber cinereum, The infundibulum, The hypophysis.
The lamina terminalis, or end plate, rises as a thin sheet in front of the chiasma opticum and continues in front of the commissura anterior and the columnae fornicis. Between it and the chiasma lies the recessus opticus. The thin lamella originally formed the middle part of the front wall of the fore-brain ; later it is displaced, lies more deeply, and then forms the anterior wall of the third ventricle, in whose roof-plate it is continued.
The chiasma opticum forms a white quadrangular plate, from the anterior cor ners of which proceed the nervi oftlici and from the posterior corners the tractus The latter run as flattened cords outward and backward along the hind border of the substantia perforata anterior ; after passing around the pedunculi cerebri and, farther along, above and somewhat lateral to the uncus gyri hippocampi, they lead into the region of the metathalamus.
The tuber cinereum lies behind the chiasma, bounded laterally by the optic tracts and the cerebral peduncles and behind by the corpora mamillaria. This gray elevation is a thin lamina and assists in forming the floor of the third ventricle. Traced forward, it passes into the lamina terminalis, and in this anterior position is pushed into the ventricle by the chiasma. Below, the tuber cinereum is continuous with a hollow
funnel-shaped structure, the infundibulum, whose cavity is known as the recessus infun dibuli. To the end of the infundibulum is attached the hypophysis cerebri or pituitary body, a gray mushroom-shaped structure, about the size of a bean, whose longest diam eter is placed transversely.
The hypophysis, on being sectioned, is seen to be composed of a larger anterior and a smaller posterior lobe. Genetically, only the posterior lobe belongs to the brain, as a ventral evagination from the diencephalon. The lobus anterior originates as an evagination from the embryonal oral recess. In consequence of constriction and isolation, the evagination later gives rise to the hypophysial vesicle, which subsequently trans forms into the gland-like structure that, as the anterior lobe, becomes united with the lobus posterior.
Further, at particular points of the tuber cinereum, one often notes small evagina tions. One, located mostly medial and immediately in front of the corpora mamillaria, has been named by Retzius the eminentia saccularis, while the smaller and lateral eleva tions are the eminentiae lalerales. In Fig. 3o the eminentia saccularis is plainly seen. It represents, perhaps, a rudiment of the saccus vasculosus strongly developed in the bony and cartilaginous fishes.