Kappers, Ttdeosteajt and Selachian Brain. 63 



diately under the ventricle important systems of fibers originate 

 and terminate. 



The course of the optic fibers I have already mentioned w^hen 

 treating of the 'tween-brain, indicating both their division into 

 two roots, a dorsal and a ventral, about the corpus geniculatum 

 laterale and their termination in the upper layer of the tectum. 

 Though with much less certainty than in the teleosts where the 

 degenerated condition of one N. opticus proved very useful to me, 

 I was, nevertheless, able to see in Galeus also the fibers of the 

 tectum situated directly mesial to the optic fibers terminating in 

 the corpus geniculatum laterale as hrachia tecti. 



For the finer structure of the ganglion cells and their mutual 

 relations I am obliged to refer to Houser, who examined them by 

 the GoLGi method, and proceed at once to the description of the 

 deep medullated layer, the stratum album projundum. Here, as 

 in the teleosts, there is a lamina commissuralis tccti, a flat but very 

 large dorsal commissure of the two parts of the tectum (clearly to 

 be seen in Fig. Iv). As in the teleosts, the most anterior fibers 

 of this commissural system constitute a part of the commissura 

 posterior (Fig. Iv), of which by far the greater part consists of 

 fibers which disappear in the cell layers which are situated imme- 

 diately under the optic ventricle (Fig. Ivi). 



Before continuing the description of the tracts I must mention 

 that the diff"erentiation of this gray substance under the optic 

 ventricle is by no means so clear as in the teleosts, either in the 

 outlines of this region or in the distribution of the ganglion cells. 



The eminentice medtales are scarcely visible and the only thing 

 to be stated here about the torus semicircularis is that in Galeus 

 I found a distinct but small protrusion of the postero-lateral sub- 

 tectal region of the mid-brain (Fig. lix), which I must suppose to 

 be the homologue of the colliculus, or torus semicircularis. That 

 this is the case I conclude (i) from its situation in the posterior 

 subtectal region; (2) from the fact that, as in the teleosts, it is 

 provided with numerous ganglion cells (^nucleus lateralis mesen- 

 cephali); (3) from the fact that there is a very distinct medullated 

 tract which arises in the stratum album profundum tecti and 

 terminates here. This tectal relation was also found in the tele- 

 osts, where its fibers were situated between those lemniscus fibers 

 which pierce through the colliculus. This is not the case here 

 because the situation of the colliculus in the selachians is more 



