a. Thalamus ([Fig. 141], t).—The two thalami are large oblique rounded ridges, forming the sides of the ’tween-brain. They lie just craniad of the cranial corpora quadrigemina (q), but separated from them by a broad groove, and are completely covered by the caudally projecting part of the cerebral hemispheres. The medial larger end of each thalamus is near the middle line; thence the thalamus extends caudolaterad, and rises at its lateral extremity into a sharply rounded projection, the corpus geniculatum laterale (x). From this projection the thalamus is continued ventrad and then craniomediad as a large band of fibres, the optic tract (tractus opticus) ([Fig. 142], d), which extends to the optic chiasma (e), where it passes into the optic nerves (f). On the caudal border of the thalamus, near the median line, is a very faint projection, the pulvinar ([Fig. 141], w); this lies just craniad of the lateral border of the cranial corpus quadrigeminum (q). Just ventrad of the corpus geniculatum laterale (x) is the prominent rounded corpus geniculatum mediale (s); this is connected by a prominent ridge, the brachium quadrigeminum inferius (r), with the caudal corpus quadrigeminum (p). In a similar manner the brachium quadrigeminum superius passes from the cranial corpus quadrigeminum (q) into the thalamus itself.
Between the two thalami there exists a groove, the sulcus hypothalamicus. Over this groove lies the roof of the third ventricle, forming the choroid plexus of the third ventricle ([Fig. 143], w). The medial surface of the two thalami are flat and extend directly ventrad, forming part of the lateral boundary of the third ventricle ([Fig. 143], h). The two medial surfaces meet over a considerable area across the narrow cavity of the third ventricle, and unite, forming the massa intermedia ([Fig. 143], f) or intermediate mass of the thalamus (“middle commissure”). This connection of the thalami of the two sides is thus not a primitive one, forming no part of the roof or floor of the central cavity, but is a secondary connection due to a growing together of a part of the two sides of the ventricle across its cavity. Along the dorsal edge of the medial border of each thalamus passes a distinct white strand, the stria medullaris ([Fig. 141], u); the two striæ meet in an arch caudad, lying beneath the pineal body (2).
The thalamus is separated craniolaterad by a groove (1) from the corpus striatum (y), on the floor of the cerebral hemisphere.
b. The roof of the third ventricle is thin and united with the pia mater, as already stated. The pia mater bears many blood-vessels, and the two are folded into the groove between the optic thalami, forming the lamina chorioidea epithelialis, or choroid plexus of the third ventricle ([Fig. 143], w). The roof is attached to the dorsomedial borders of the thalami and becomes continuous with the choroid plexus of the lateral ventricles ([Fig. 148], e) at the craniolateral borders of the thalami.
The pineal body (corpus pineale) or epiphysis ([Fig. 141], 2; [Fig. 143], y) is a small conical body which is formed as an outgrowth of the caudal part of the roof of the third ventricle. It lies on the roof of the brain between the two cranial corpora quadrigemina ([Fig. 141], q). It is hollow, containing an extension of the third ventricle. From its craniolateral angles two white strands, the habenulæ, extend into the striæ medullares ([Fig. 141], u) of the thalami.
Just ventrad of the pineal body is a transverse band of white fibres, lying in the caudal part of the roof of the third ventricle. This interconnects the two thalami, and forms the posterior commissure ([Fig. 143], z′) (commissura posterior). From this commissure a thin sheet of tissue extends to the pineal body.
c. The floor of the third ventricle appears in a ventral view of the brain ([Fig. 138], [Fig. 142]) as a somewhat diamond-shaped space craniad of the pedunculi cerebri ([Fig. 142], a) and bounded along its cranial margin by the optic tracts ([Fig. 142], d). The optic tracts come from the thalami, as already [described]; they converge and unite to form the optic chiasma ([Fig. 142], e), from which the optic nerves (f) diverge.
Immediately caudad of the optic chiasma lies a considerable rounded gray elevation, the tuber cinereum ([Fig. 142], h). This bears on its ventral surface in the natural condition the infundibulum ([Fig. 138], d) with the hypophysis ([Fig. 138], e); in cases where the two latter structures have been removed ([Fig. 142]) the tuber cinereum (h) bears a small longitudinal opening (i) for attachment of the infundibulum. The infundibulum ([Fig. 138], d) is a hollow extension of the floor of the third ventricle, and is attached to the middle of the ventral surface of the tuber cinereum. It bears at its ventral end the hypophysis (pituitary body) ([Fig. 138], e), a vascular non-nervous body of unknown function. The hypophysis is lodged in the sella turcica of the sphenoid bone.
At its caudal border the tuber cinereum bears two white elevations, the mammillary bodies (corpora mammillaria) ([Fig. 142], g).
