354 NERVOUS SYSTEM. 
covered by the caudally projecting part of the cerebral hemi- 
spheres. 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 (7). From this projection the 
thalamus is continued ventrad and then craniomediad as a large 
band of fibres, the optic tract (tractus opticus) (Fig. 142, 2), 
which extends to the optic chiasma (¢), where it passes into the 
optic nerves (/). 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 (g). Just ventrad of the corpus genicu- 
latum laterale (x) is the prominent rounded corpus geniculatum 
mediale (s); this is connected by a prominent ridge, the 
brachium quadrigeminum inferius (7), with the caudal corpus 
quadrigeminum (f). In a similar manner the brachium 
quadrigeminum superius passes from the cranial corpus quad- 
rigeminum (¢) 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, “). 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, /) 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 con- 
nection 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, ~); the two striz 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. 
