EXAMINATION OF TBB ENCEPHALIC SEGMENTS. 453 



ental aspect of a delicate membranous roof of the diacoelia, the 

 diatela (Fig. 122). 



By slightly pushing the hemisphere away from the thalamus, 

 it will be seen that the diatela springs from the dorsal margin of the 

 habena, along a slight furrow, the Sulcus habenae {81. h.\ and 

 curves dorso-mesad to meet its other half from the opposite side. 



§ 1144. On the lateral aspect, between the cimbia and the tractus 

 opticus, note a decided elevation, the postgeniculatum (Fig. 18 ; 

 PI. Ill, Fig. 7, 9, pgn.). Ventrad of it is a ridge of the cms, partly 

 embracing a depressed area, the quadrans (Fig. 18 ; PI. Ill, Fig. 

 11, q.). 



Ventriduct the parts stiU more, and trace the tractus opticus 

 into an expanded elevation just cephalad of the postgeniculatum. 

 This, the praegeniculatum (PI. Ill, Fig. 7, 9, prgn.), is practically 

 continuous with the tJialaTnus, the principal part of the diencepha- 

 lon, which has been seen above as the lateral wall of the diacoelia. 

 Note the absence of any distinct caudal protrusion of the thalamus, 

 such as forms the human pulvinar. At the meson, in the depres- 

 sion between the thalamus and the opticus, note the half of the 

 conarium, more or less enveloped by the velum, and inclined cau- 

 dad from its attachment so as to rest upon the opticus. 



By an incision beginning just dorsad of the callosum and ex- 

 tending laterad and very slightly dorsad, remove the dorsal portion 

 of the hemisphere. The features of the cut surface may be disre- 

 garded here ; the object is to permit what is left to be raised more 

 easily. On lifting this portion, note that its ventral surface presents 

 slight fissures and strise, trending latero-caudad. This is the fornix 

 (PI. IV, Pig. 14,/.), including the lyra {ly.), the limits of which have 

 not been determined. Note that, for a short distance from the sple- 

 nium, the plane of the fornix coincides with that of the callosum, 

 but that it curves ventrad so as to become nearly vertical at its 

 cephalic end. The general shape of the fornix is triangular ; its 

 larger portion is the body, and the narrower cephalic end is the col- 

 umna. The band which forms its lateral border is the fimbria, 

 which, as will be seen later, is continued nearly to the tip of the 

 LI. hypocampcB. 



Between the fornix and the diencephalon is a fold of pia, the 

 velum. When freed from the velum, note the difference between 

 the dorsal surface of the thalamus and the mesal surface already 



