Herrick, Morphology of Brain of Bony Fishes. 43 



the fibres are forced to make a long detour. In entering the 

 torus they dip through all the other strata of the tectum. See 

 Figs. I and 4, Plate VI, Figs. 6 and 9, Plate VII, and Fig. 8, 

 Plate VIII for examples. A considerable part of the fibres 

 which enter the torus longus are not nerve fibres, but arise from 

 the modified epithelium of that body and pass to the ectal sur- 

 face of the tectum. Such gelatitious tracts are to be regarded as 

 mere persistent walls of the original single layer epithelium and 

 add proof, if it were necessary, that the torus is but a part of the 

 tectum. The greater number of such' fibres form a support for 

 the mass of optic fibres where the brachium cephale enters the 

 tectum (Plate VII, Fig. 9.) 



A third system belongs to the thalamus and its commissural 

 tracts. It is chiefly limited to the cephalo-dorsal part of the 

 tectum and its fibres collect above the granular layer mto 

 small circular bundles which in transverse section are transverse- 

 ly cut and in longitudinal section lie in the plane of section. 

 These bundles collect ventro mesad of the cephalic optic tract 

 fibres and pass with them to near the level of the habena, thus 

 constituting the origin of the cephalic brachium. They are dif- 

 fused in the same nidulus from which the transverse commissure 

 fibres arise. It also appears that the commissura ventralis sus- 

 tains the same relation to the caudal optic tract, except that the 

 commissural fibres are more closely associated with those of the 

 optic tract than in the previous case. That part of the ventral 

 commissure tract which is derived from the cinereum, however, 

 passes into the torus semicircularis (coUiculus). 



If the above relations are correctly made out, tho essential 

 structure is simple indeed. There is morphologically a single 

 ventral commissural system related to the optic system and to 

 the brachia. The same cause which has operated to divaricate 

 the two portions of the optic tract may have separated the ven- 

 tral system. There is apparently an interruption and cellular 

 interpolation in both divisions. 



The caudal system remains to be considered. Collectively 

 these fibres constitute a strong bundle passing through the col- 

 liculus from their origin in the ganglion layer of the tectum ectad 

 of the granular zone. These fibres enter the colliculus in con- 

 centric order and there divide into several sheathing concentric 



