SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 221 



In such sections it would be very difficult to determine the true course 

 and relations of these axons, even if they were corajjletely impregnated. 



P. Cajal ('99^ and Catois (:0l) follow Sala closely in their interpre- 

 tation of the torus longitudinal is. Catois (p. 130) describes, in addition, 

 certain nerve-fibres which terminate in the torus, and, as he believes, 

 are derived in part from the dorsal decussation and in part from the third 

 layer of the tectum. These fibres end in connection with the dendrites 

 of the torus-cells. Johnston in reviewing the literature of this subject 

 says (:03, p. 1076) : " Dass der Torus longitudinalis eine besondere 

 Bedeutuug habe, ist noch nicht endgilltig gezeigt worden. Sala uixd 

 Catois geben ihn als die Ursprungsstelle der centrifugalen Fasern in den 

 optischen Nerven an, aber wie Referent nachgeweisen hat, sind diese 

 Fasern bei Wirbeltieren vorhanden, bei denen der Torus fehlt. Seine 

 gegenwart wird wahrscheinlich durch den Einfluss des oben beschrieben 

 nucleus magnocellularis tecti veranlasst." 



E. Summary for Tkleosts. 



The midbrain roof of teleosts is more complex in its structure than in 

 the groups previously described. Projecting downward from the median 

 roof of the mesencephalon is the torus longitudinalis, a paired ridge ex- 

 tending along the median roof of the optic lobes, and ontogenetically 

 derived from the mesal edges of the tectum. Phylogenetically the torus 

 has its beginning in the ganoids, the increase in size of the optic reflex 

 cells and the pressure of other surrounding portions of the brain result- 

 ing in a down-swelling at the anterior end of the tectum forming two 

 incipient lobes or ridges on either side of the median plane. In the 

 primitive Siluridae and degenerate Amblyopsidae there is only a slight 

 advance upon this condition. In the Salmonidae the torus lobes are 

 laterally compressed and the median fissure obliterated by the pressure 

 between the halves of the thick tectum. In nearly all other teleosts the 

 torus-lobes hang suspended freely and more or less separately within the 

 ventricle, assuming a considerable variety of forms in the different groups. 



The cells of the optic reflex apparatus, in common with the other 

 cellular elements of the brain, are of relatively small size in the teleosts, 

 and are concentrated in the torus longitudinalis. Unipolar or bipolar 

 in their general outline, they ultimately give rise to three processes 

 forming three fibre-tracts on each side of the brain. The fascicles of 

 fibres of the tractus toro-tectalis and tractus toro-cerebellaris running 

 dorsad and cephalad pass out of the torus together into the tectum. 

 The former tract crosses through the dorsal decussation and continuing 



