THE CENTRAL NERVOUS SYSTEM. 355, 



wliich extend farther craniad than this are lateral portions, due 

 to the forward growth of the lateral hemispheres. The two 

 hemispheres of the cerebrum may be considered as lateral out- 

 growths of the central 'tween-brain ; these outgrowths have 

 extended dorsad, latcratl, craniad, and caudad, so as to cover 

 almost completely the 'tween-brain. 



In early stages the cerebral hemispheres are projections from 

 the cranial end of the 'tween-brain, so that the plane of junc- 

 tion was nearly transverse, the cranial end of the 'tween-brain 

 joining the caudal end of the hemispheres. With the increas- 

 ing size and backward growth of the latter, the attachment to 

 the 'tween-brain has been shifted from a cranial to nearly a 

 lateral position, and at the same time the originally lateral sur- 

 face of the 'tween-brain has become nearly caudal. This is 

 shown in I'^ig. 141, the deep fissure at i marking the line of 

 attachment between the 'tween-brain and the hemispheres. 

 The dorsoventral plane of junction of 'tween-brain and hemi- 

 spheres is (as l-'ig. 141 shows) not wholly lateral, but oblique, 

 passing from its cranial end near the middle line caudolaterad. 



A second peculiarity of the 'tween-brain lies in the thinness 

 of its roof. The roof is exceedingly thin and is so intimately 

 connected with the pia mater that they cannot be removed 

 separately. The ventral thick floor of the 'tween-brain is 

 directly continuous with the similar floor of the cerebrum ; but 

 where the roof of the 'tween-brain joins the roof of the cerebrum 

 along the oblique plane already indicated, the roof is very thin 

 and is intimately connected to the pia mater, and is at the 

 same time folded into the lateral ventricles to form, together 

 with the pia mater, the choroid plexuses of the lateral ventricles 

 (Fig. 148, e). When the pia mater is removed the thin roof of 

 the brain along the line of junction of the 'tween-brain and the 

 cerebrum is brought away and there appears to be a direct 

 communication between lateral ventricles and the exterior. 



We may now take up the parts of the diencephalon in detail. 



a. Thalamus (Fig. 141, /). — The two thalami are large 

 oblique rounded ridges, forming the sides of the 'tween-brain. 

 They lie just craniad of the cranial corpora cjuadrigemina {q), 

 but separated from them by a broad groove, and are completely 



