SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 169 



rites are coarse, sharply defined, and unmedullated (Fig. 18). - They are 

 collected into fascicnli, in which the aepai'ate fibres may be distinctly 

 followed (Fig. 15). These fibre-tracts run caudad through the tectum, 

 on either side of the median plane, and at some distance from it (Plate 2, 

 Fig. 12). Near the posterior limit of the tectum these two lateral fibre- 

 tracts, still composed of many fasciculi in which the separate fibres are 

 clearly distinguished, converge, bend downward, and passing under the 

 trochlearis commissure, and through the narrow bridge of connecting 

 tissue enter the cerebellum (Plate 3, Fig. 15). A part at least of these 

 fibres decussate in the base of the cerebellum (Fig. 15, dec), passing to 

 the opposite side of the brain from that in which they arise. The fibres, 

 here separating, ascend and distribute themselves through the basal fibre- 

 layer of the cerebellum (Fig. 15). In this anterior median portion of 

 the cerebellum the granular layer is absent or thin, and the Purkinje 

 cells are in direct contact with this fibre-layer. The fibres of the tractus 

 tecto-cerebellaris probably end in fibrillations in the molecular layer of 

 the cerebellum. 



The thii'd set of nenrites from these cells, which for distinction I have 

 called the axons (Figs. 15, 18, ax'"), pass by more or less direct paths 

 to the anterior end of the mesencephalon, where they enter the ventricle 

 in a manner yet to be described. The axon is finer and more delicate 

 than the cei'ebellar neurite, and moi'e difiicult to follow. These axons 

 may arise from the cell independently, or by division of the single nerve- 

 process. From the posterior cells the axons pass cephalad, turning 

 slightly laterad and dorsad (Fig. 18, a.r'") to form two loosely aggre- 

 gated fibre-tracts, which run forward through the tectum on either side 

 the median plane (Fig. 12, trt. tct.fhr. Reis.). From the middle cells of 

 the series the axons usually arise by a S[)litting of the T-shaped process, 

 and pass laterad and dorsad, joining the lateral fibre-tracts which run 

 anteriorly. This tractus tecto-fibrae Reissneris runs iumiediately below 

 the dorsal decussation (Figs. 15, IG) somewhat dorsal and mesal of 

 the tractus tecto-cerebellaris (Fig. 12). In the anterior portion of the 

 tectum these two fibre-tracts converge toward the median plane, the 

 separate axons meanwhile uniting into fascicles and becoming more 

 ch)sely consolidated. The bundle of fascicles thus formed continues ven- 

 trad as far as the level of the posterior commissure, where it curves 

 posteriori}' and enters the ventricle to form Reissner's fibre (Fig. 16). 



The exact method by which the fascicles enter the ventricle and form 

 Reissner's fibre has been difficult to make out completely in Raja, the 

 connections having been broken away in all my series of sections. In 



