114 

 order. Tney are cert-iinly of far less importance as an optic 

 termination in the selachian than are their specialized repre- 

 sentatives in the mamnnal. In y.ustelas, only collateral branch- 

 es are, evidently, sent to the nucleus ^eniculatum, the great 

 mass of optic fibres sweeping backward to the midbrain for ter- 

 mination in the tectjm. 'flith the progressive evolution of 

 higher vertebrates, the thalamic termination of the opticus 

 appears to have become more and more important, leading to the 

 corresponding differentiation of geniculate nuclei. Hand in 

 hand witn the growing importance of the interbrain as a prima- 

 ry optic centre, however, there has been a related decline of 

 the midbrain roof. And hence it is that the optic lobes of 

 the fish appear so lisproportionately large in comparison with 

 the homologous parts of the mammalian brain. 



2. Epi thalamus: The Nuclei Eabenulae. 



The nuclei habenulae, or ganglia habenulae of authors, 

 rise considerably above the level of the thalami (Pig.l, n.h.), 

 the pair meeting each other to form a conspicuously arched 

 bridge across the third ventricle at its posterior end. The 

 epiphysis springs from the middle of the arch. The left nucle- 

 us is a little larger than the right one, its margin extending 

 slightly more anteriorly. 



The nucleus habenulae is an important relay-centre, and so 

 its strjctjre exhibits many nerve-fibres taking various direc- 



