8 Journal of Comparative Neurology. 



line and expands into the epiphysis dorsal to the fore brain. In 

 the choroid roof of the 'tween- and fore -brain, immediately ven- 

 tral to the stalk of the epiphysis, there extends forward from 

 the left ganglion habenulae a bundle of fibers which expands 

 into a grey mass beneath the epiphysis. This is the Zirbelposter 

 of Ahlborn (^'^l). 



Looking again at the ventral portion of the 'tween-brain, 

 it is seen that in transverse sections in the plane of the superior 

 commissure the lateral walls fuse together in such a way as to 

 close off the ventral part of the cavity from the third ventricle. 

 This part of the cavity of the inferior lobes is triangular in 

 transverse section and extends forward to the optic chiasma 

 beneath the caudal border of which it ends blindly, forming 

 thus a very deep postoptic recess (Fig. 19). The tissue which 

 roofs over this recess contains the large postoptic decussation, 

 in front of which the optic tracts decussate before leaving the 

 brain. 



In sections through the. optic chiasma no other part of the 

 'tween-brain is present, but instead the sections pass through 

 the body of the fore-brain. The lateral lobes, or so-called 

 hemispheres, of the fore-brain are seen at the sides of the 

 'tween-brain in sections considerably caudal to the chiasma 

 (Figs. 18, 19). In sections just in front of the chiasma the 

 rounded cavities of these lobes communicate widely with the 

 central cavity. Below this point of communication the lateral 

 walls constitute the corpora striata in the narrow sense, between 

 which the cavity is produced ventrally into a characteristic pre- 

 optic recess surrounded by the substance of the nucleus 

 thaeniae. Above the lateral expansions of the cavity appears 

 on either side a prominent ridge which forms the dorsal part of 

 the lateral wall of the fore -brain from a point a little in front of 

 the ganglia habenulae to the olfactory commissure (Figs. 17, 

 18). This is the epistaiatum (optic thalamus and striatum of 

 Ahlborn, thalamus (?) of Friedrich Mayer, '97). The lateral 

 lobes above mentioned are divided externally by a slight groove 

 into anterior and posterior parts (Fig. i), and internally the 

 lateral expansions of the cavity divide into anterior and poste- 



