SAEGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 195 



In Acipenser they are but slightly developed, but have been noticed by 

 Johnston (:01, Fig. 20). In adult Lepidosteus they are somewhat more 

 pronounced and were observed and figured by C. L. Herrick ('9l). 



The cells are multipolar, giving oflf two principal processes in addi- 

 tion to the axon. One of these processes has been traced dorsad and 

 caudad near the median plane into the middle fibre layer of the tectum, 

 whence it probably bends ectad and comes into contact with the affer- 

 ent nerve-fibres of the optic tracts. The optic tracts coming from the 

 thalamus enter the tectum at its anterior border (Plate 6, Figs. 40-42, 

 tH. opt.\ and run posteriad through the ectal portion of the tectum 

 near the median plane, giving ofi" their fibres in bundles to break up in 

 the outer third of the tectum. The second process of the optic reflex 

 cells passes ectad into the superficial portion of the tectum and enters 

 a tract running through the lateral wall of the tectum and into the 

 cerebellum, thus establishing connection for the co-ordination of the 

 muscular movements to which the apparatus gives rise. 



Through the distal portion of the canalis centralis Eeissner's fibre 

 bi'eaks up into its constituent axons, Avhich pass mto the ventral part 

 of the cord, and probably out through the ventral roots to the muscu- 

 lature. In the extreme posterior portion of the canal, and in the ven- 

 triculus terminalis, there are from five to ten posterior canal-cells, 

 derived from the walls of the cord, which send their axons cephalad 

 through the canal and joining Eeissner's fibre become a pai-t of it. 

 These cells are in intimate connection with the cord by numerous 



processes. 



IV. Teleosts. 



The teleost brain presents a form of great complexity, considerably 

 removed from the more generalized types found in ganoids, selachians, 

 and amphibians. This large and variable group presents many types of 

 brain structure, and in common with this the morphology of the mes- 

 encephalon varies greatly. Consequently, the optic reflex apparatus 

 undero-oes consideraV)le modification in the nature of its constituents 

 and in its relations to the surrounding brain structures. This makes 

 a broad and comprehensive study of the apparatus in this group desira- 

 ble, and with this in view I have examined "all the fish that came to 

 my net," representing thirty families and about sixty species. 



A. Historical. 



Though the nervous system of teleosts has received so much attention 

 from comparative neurologists, Eeissner's fibre has been noticed by only 



