NERVOUS SYSTEM. VERTEBRA TA. 85 



extends within the hemisphere to the olfactory bulb. The 

 posterior part of this cavity is mainly occupied by a 

 prominent swelling on the lateral wall of the hemisphere, 

 that probably represents the corpus striatuin. 



The thalamencephalon forms a tube 25 mm. long ; its 

 sides are ribbon-like and fairly thick, but the floor and roof 

 are composed only of epithelium and pia mater. The 

 roof is more extensive than the space between the lateral 

 walls and thus bulges upwards, particularly in front, in a sac- 

 like manner. It is infolded in the mid-line in its anterior 

 two-thirds to form a choroid plexus, small processes of 

 which project into the lateral ventricles. At its hinder end 

 it forms a conical projection that accompanies the epi- 

 physis. In front of the epiphysis the roof of the thalamen- 

 cephalon is swollen to form a pair of prominent ganglia 

 habenulse which, as in the Amphibia, form a projecting 

 lip when the tela choroidea is removed (upper specimen). 

 Behind the epiphysis, the anterior border of the optic 

 tectum is thickened to form the posterior commissure. 

 The optic lobes are of considerable size, and contain a large 

 ventricle, the lower half of which is partly filled by a swelling 

 of the lateral walls. The cerebellum is simple, though well 

 developed: it is diamond- shaped, with anterior and posterior 

 lobes each containing an offshoot from the common ventricle; 

 its surface is smooth with the exception of a longitudinal 

 furrow and a feeble transverse indentation. The inner 

 surface of the cerebellum is marked by a pair of prominent 

 longitudinal ridges similar to those of Elasmobranchs, and, 

 like them, continuous with the medullary auricles. 



The medulla is strongly concentrated antero-posteriorly, 

 and is remarkable for the great development of two pairs 

 of lobes. One pair (lobus lineai lateralis) is continuous 

 with the auricles and gives origin on either side to the 

 dorsal root of the ophthalmicus superficialis vn. The 

 second pair (tuberculum acusticum) lie below them, and 

 give rise to the lateralis and the lower roots of the lateral- 

 line branches of the vil. nerve. The lobus vagi is nodulated 

 as in Elasmobranchs. The lobi inferiores, though small, 

 are remarkably prominent. The saccus vasculosus is well- 

 developed. 



