THE TELENCEPHALON 783 
glandular in character and there is good ground for the view that it is an organ of internal se- 
cretion. Besides the chief cells, which stain lightly, it contains large, deeply staining chromophile 
cells. It arises-as an outgrowth from the dorsal wall of the primitive mouth cavity. The cerebral 
lobe is pale and is connected with the infundibulum so as to form a rather flask-shaped arrange- 
ment. It is almost entirely enclosed by the glandular part. It arises as an outgrowth from the 
primitive diencephalon, but loses most of its earlier nervous character. 
The optic chiasm and tracts form the anterior boundary of the interpeduncular 
fossa (Figs. 629, 636). The optic chiasm (Chiasma opticum) is formed by the 
convergence of the optic nerves and the crossing of the major part of the fibers of 
the nerve of one side to the tract of the opposite side. From the chiasm each optic 
tract (Tractus opticus) curves obliquely around the cerebral peduncle to the pos- 
terior part of the thalamus and the medial geniculate body; some fibers reach the 
anterior quadrigeminal body. 
All the fibers in the chiasm are not derived from the optic nerves. The posterior part 
contains fibers which pass from one tract to the other and are connected with the medial genicu- 
late bodies; this bundl> is called the ventral commissure (Commissura ventralis). Above it is 
the dorsal commissure (Commissura dorsalis), the fibers of which enter the hypothalamic body.1 
The third ventricle (Ventriculus tertius) is the narrow annular space between 
the thalami. It communicates by means of the cerebral aqueduct with the fourth 
ventricle behind, and in front it is continuous with the lateral ventricle on each side 
through the interventricular foramen. Its floor is formed by the structures of the 
interpeduncular fossa, and to a small extent by the tegmentum of the cerebral 
peduncles. The roof is formed in the strict sense only by the ependyma, above 
which is a fold of pia mater, termed the tela chorioidea of the third ventricle.2, The 
roof is invaginated by the two delicate chorioid plexuses (Plexus chorioidei ventriculi 
tertii) which appear to lie within the ventricle, although they are excluded from the 
cavity by the epithelium. When the tela is removed, the delicate ependyma of the 
roof is torn away with it, leaving the line of attachment to the stria medullaris to 
constitute the tenia thalami. The anterior wall is formed by the lamina terminalis, 
a thin layer of gray matter which extends from the optic chiasm dorsally to the 
corpus callosum. A distinct rounded band of white matter extends across its 
posterior face, bulging into the ventricle. This is the anterior commissure (Com- 
missura oralis) of the cerebrum; its fibers extend to the olfactory bulb and to the 
piriform lobe. A similar but more slender posterior commissure (Commissura 
aboralis) crosses the posterior wall above the entrance to the cerebral aqueduct; 
the connections of its fibers are not yet clearly known. The interventricular 
foramen’ is situated on either side of the anterior part of the ventricle, and leads 
outward and slightly dorsally between the column of the fornix and the anterior 
tubercle of the thalamus (Fig. 639). The cavity presents three recesses or diver- 
ticula, of which two are ventral and the third is posterior. The optic recess (Re- 
cessus opticus) lies above the optic chiasm. Just behind it is the infundibular 
recess (Recessus infundibuli), which extends through the infundibulum to the 
pituitary body. The pineal recess (Recessus pinealis) is in the stalk of the pineal 
body. 
THE TELENCEPHALON 
The telencephalon or end-brain comprises two principal parts, the cerebral 
hemispheres and the optic part of the hypothalamus. The latter has been con- 
sidered as a matter of convenience in the description of the diencephalon. 
THE CEREBRAL HEMISPHERES 
The cerebral hemispheres (Hemisphria) form the greater part of the fully 
developed brain. Viewed from above (Fig. 631) they form an ovoid mass, of 
1These are commonly termed Gudden’s and Meynert’s commissures respectively. 
Also commonly termed the velum interpositum. 3 Also termed the foramen of Monro. 
