NERVOUS SYSTEM. VERTEBRATA. 67 



projects beak-like above the cerebrum, but posteriorly it is 

 nervous and swollen to form an asymmetrical pair of ganglia 

 (ganglia habenulse), the smaller (left) of which is in con- 

 nection with the pineal body. In the specimen these 

 ganglia can only be distinguished as a single median 

 thickening in front of the optic lobes. The ganglia habenulse 

 are one of the most constant parts of the vertebrate brain. 

 They are always present on either side close in front of the 

 epiphysial evagination, are united by a commissure, and 

 are always in connection by tracts with the olfactory area 

 of the cerebrum and with the corpus interpedunculare. 

 The latter can be seen in the specimen as a small median 

 protuberance close behind the origin of the oculo-motor 

 nerves. The floor of the thalamencephalon is expanded 

 ventrally to form a capacious infundibulum. Lobi inferiorea 

 and saccus vasculosus are absent, but there is said to be a 

 well marked saccus infundibuli in the posterior wall of the 

 infundibulum. A similar structure occurs in Elasmo- 

 branchs and embryo Teleosts. The side walls consist 

 mainly of tracts that connect the cerebrum with the more 

 posterior regions of the brain. They are covered super- 

 ficially by the optic tracts on their way downwards and 

 forwards from the optic lobes to the chiasma in front of the 

 infundibulum. The thalamencephalon is continuous at its 

 anterior end, on either side of the mid-line, with a pair of 

 hollow lobes the secondary fore-brain each consisting of 

 two parts : a posterior lobe (the hemisphere) , and a larger 

 anterior lobe (the olfactory bulbj from which a nerve is 

 given off to the nose. A slight lobulation observable upon 

 the surface of the olfactory bulbs is due to a glomerular 

 condensation of the neuropile close beneath the surface. 



The hemispheres are mesially united by a commissure 

 (anterior commissure) that lies in the dorsal parts of the 

 anterior wall of the thalamencephalon (lamina terminalis). 

 They have ganglion-cells scattered irregularly throughout 

 their walls and show no signs of cortical structure. 



Ahlborn, Zeits. wiss. Zool., Bd. xxxix. 1883, p. 191. 



