126 



ANATOMY OF THE CENTEAL NEEVOUS SYSTEM. 



system of the optic tract, which passes down to the chiasma from the roof 

 of the midbrain. 



The best starting-point is the very simply constructed thalamen- 

 cephalon of the amphibian. It is an elongated body of oval cross-section, 

 continuous anteriorly with the prosencephalon and posteriorly with the 

 mesencephalon. Close behind the plexus chorioides of the cerebrum is 

 located on either side a small ganglion, — Corpus habenulw, — and this 

 division is designated the epithalamus. On the ventral side lie several 

 prominences and aggregations of ganglia which are, in part, readily dis- 

 tinguished from the rest of the thalamus, and are grouped together as the 

 Hypothalamus. The principal body which lies between these two just 



•COO 



Fig. 75. — Section through the Dieneephalon of Bufo: toad. 



named retains the name Thalamus. These three divisions are practically 

 demonstrable in all vertebrates, but the epithalamus alone is uniform in its 

 structure, the other divisions varying much in different genera. 



The central cavity of the thalameneephalon is closed dorsally with 

 several folds of the same epithelial plate as had formerly, constituted the 

 entire encephalon (see Figs. 18 and 20). Besides this, the roof contains the 

 fibers of a small commissure: Commissura habenularis. 



Anteriorly the interbrain is separated from the cranial cavity by the 

 Lamina terminalis (see Chapter IV, page 49). It is always narrow, and on 

 either side of it an opening leads into the central cavity of the hemispheres, 

 which even in the embryonic period are evaginated dorso-laterally from 

 this location. 



