114 



NERVOUS SYSTEM 



third ventricle). The pineal body is at first a simple diverticulum of the cavity 

 of the fore-brain. 1 Its ependymal lining becomes thrown into folds, and the cavity 

 is ultimately obliterated. The stalk remains patent, and forms the pineal recess: 

 It is at first directed upwards ; but as the hemisphere grows backwards the gland 

 is thrown over on to the corpora quadrigemina, and the stalk assumes the curvature 

 seen in the adult brain. The prominent rounded swellings seen on the sides of 

 the fore-brain (fig. 150) are the rudiments of the lateral geniculate bodies. A section 

 (fig. 164, p. 120) shows that the external prominences correspond to diverticula 

 of the cavity of the vesicle. The area on each side, between the geniculate promi- 

 nence, the superior brachium, and pineal peduncular ridge is the habenular region. 

 When the hemisphere grows back over the thalamus (see below), the geniculate 



h emispJi ere-vesicle 



tongue 



FIG. 157. SECTION OF THE CEREBRAL HEMISPHERES FURTHER FORWARDS THAN THE SECTION 

 GIVEN IN FIG. 156. Photograph. (T. H. Bryce.) 



The section should be compared with the model represented in fig. 150. The mesenchyme in the- 

 great longitudinal fissure between the hemispheres is the primitive falx cerebri ; the choroidal fold is 

 in process of formation. On the left of the figure he corpus striatum is divided by a furrow into two- 

 limbs (see floor of right hemisphere in fig. 150) 



angle is displaced backwards to its definitive position, and the habenular an 

 forms the trigonum habenulce and the pulvinar. The grey matter in the habenulai 

 area becomes the ganglion habenulce, and the two areas are early connected by 

 commissure across the roof in front of the epiphysis. When the stalk of that bodj 

 is folded back, the crossing fibres are found in the dorsal lamella of its peduncle 

 This habenular commissure must not be confused with the posterior commissure 

 which forms at the junction of mid- and fore-brain below and behind the posterior, 

 afterwards the ventral, lamella of the pineal stalk. The mesial geniculate bodie 



1 In lower vertebrates the pineal diverticulum is bilateral, but only that of the left side devel< 

 into the pineal gland (Cameron). 



