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 hghtly, it contains large, deeply staining chromophile 

 ceUs. 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 earher 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 obUquely around the cerebral peduncle to the pos- 

 terior part of the thalamus and the medial geniculate body; some fibers reach the 

 anterior quadrigeminal body. 



AH 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 bundlo is called the ventral commissure (Commissura ventralis). Above it is 

 the dorsal co mmi ssure (Commissura dorsahs), the fibers of which enter the hypothalamic body.' 



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.^ 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 taenia thalami. The anterior wall is formed by the lamina terminalis, 

 a thin layer of gra.y 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 coimections 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 colunrn 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 (Hemispharia) form the greater part of the fully 

 developed brain. Viewed from above (Fig. 631) they form an ovoid mass, of 

 'These are commonly termed Gudden's and Meynert's commissures respectively. 

 2 Also commonly termed the velum interpositum. = Also termed the foramen of Monro. 



