Herrick, Morphology of Brain of Bony Fishes. 55 



IX. — The colliculus and fasciculus lateralis. The colliculus is 

 a more or less hemispherical projection into the optic ventricle 

 from the ventro-caudal and mesal wall. The development of 

 torus and volvula have great influence on its form. In the 

 drum the coUiculi are very distinct and are separated by the 

 above mentioned structures very completely. The substance of 

 these bodies consists of a maze of fibres and small cells in which 

 no special tracts or niduli are noticeable. A coating of fibres 

 on the cephalo lateral surface has been called the stratum zonale, 

 but most of these fibres pass into the fasciculus lateralis, (Fritsch) 

 which is the only prominent tract rising solely in the coUiculi. 

 This large tract is very direct and unambiguous and may be fol- 

 lowed far caudad, as will be seen hereafter. Some fibres from 

 the tectum may enter the meshwork, but they are ill-defined. 

 Laterad the coUiculi are bounded by the decussating fibres from 

 tectum to cerebellum which also pass obliquely through it ; 

 caudad by the ' ' Uebergangsganglion " ; ventrad they lie ob- 

 liquely upon the crura and caudo-laterad of the thalamus centres 

 of the posterior commissure region. ^ From the very base and 

 lateral aspects of this body appear to spring the caudal brachia 

 or rather those fibres which pass with the ventral commissure 

 tract and on into the cinereum. Most of these brachial fibres 

 remain on the caudo-lateral aspects of the coUiculi ectad of the 

 of the lemniscus and reach a higher level, gradually passing be- 

 tween the lemniscus bundles into the caudal part of the collicu- 

 lus where, unlike the fore part, is a distinct mass of concentric 

 cells adjacent to the Uebergangsganglion. The best ideas of the 

 relations of the colliculus may be gathered from horizontal sec- 

 tions like Figs. 3, 4, Plate V. In Fig. 3, some fibres of the 

 fasciculus lateralis have already accumulated. Figs, i and 2 

 indicate the subsequent course of the optic tracts and brachia. 

 The fasciculus lateralis was correctly described by Fritsch, who 

 adds that it is very distinct and not to be confused with any 

 other. It ends in the formatio reticularis of the medulla. 

 Mayser considers it the continuation of the lateral column of the 

 cord. .\t the level of the tuberculum acusticum it receives large 

 accessions. He recognizes no homologue in the mammalian 

 brain, (p. 516.) 



