The posterior knob-like extremity of the thalamus is called the _pulvinar_; this, as well as the two corpora geniculata and the superior corpus quadrigeminum, is connected with the optic tract.
The _third ventricle_ (see fig. 6) is a cavity situated in the mesial plane between the two optic thalami. Its roof is formed by the velum interpositum and body of the fornix; its floor by the posterior perforated space, corpora albicantia, tuber cinereum, infundibulum, and optic commissure; its anterior boundary by the anterior pillars of the fornix, anterior commissure and lamina cinerea; its posterior boundary by the corpora quadrigemina and posterior commissure. The cavity of this ventricle is of small size in the living head, for the inner surfaces of the two thalami are connected together by intermediate grey matter, named the _middle_ or _soft commissure_. Immediately in front of the corpora quadrigemina, the white fibres of the _posterior commissure_ pass across between the two optic thalami. If the anterior pillars of the fornix be separated from each other, the white fibres of the _anterior commissure_ may be seen lying in front of them.
[Illustration: From Cunningham, _Text-book of Anatomy_.
FIG. 14.--Horizontal Section through the Right Cerebral Hemisphere at the Level of the Widest Part of the Lenticular Nucleus.]
The _pineal body_ is a reddish cone-shaped body situated upon the anterior pair of the corpora quadrigemina (see figs. 3 and 6). From its broad anterior end two white bands, the _peduncles_ of the _pineal body_, pass forward, one on the inner side of each optic thalamus. Each peduncle joins, along with the taenia semicircularis, the anterior pillar of the fornix of its own side. In its structure this body consists of tubular gland tissue containing gritty calcareous particles, constituting the _brain sand_. Its morphology will be referred to later.