SARGENT: THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 201 



not been traced directly to the cells of origin. It is probable, however, 

 that the cells of the apparatus lie wholly witliin the torus longitudinalis. 

 Small fascicles of fibres make their way from the more anterior and ven- 

 tral portions of the torus lobes into the recess between the lobes of the 

 torus and above the posterior commissure (Fig. 48). A considerable 

 tract of fibres comes from the anterior and deeper portions of each lateral 

 lobe of the torus, and bending ventrad, the two meet in the median plane, 

 and pass toward and into the ventricle anterior to the posterior commis- 

 sure (Fig. 48, trt. tor.fhr. Reis. a.). The bundle becomes more and more 

 compact as it nears the ventricle, and enters it as an apparent single 

 fibre, the chief constituent of Reissner's fibre. Though the evidence 

 is not conclusive, it seems probable that Eeissner's fibre also receives 

 elements from the ganglia habenulae, for fine filaments, which probably 

 unite with Reissner's fibre, have been found in the ventricle just below 

 the pars intercalatus (Fig. ids, prs. t'cah). 



In adult Abramis crysoleucus (roach), Reissner's fibre has a diameter 

 of 3.5 micra. It is easily traced through the canal and ventricles. In 

 tlie anterior part of the mesocoele Reissner's fibre is split into a main 

 trunk and a number of finer branches. The latter emerge from the 

 roof of the ventricle, some anterior and some posterior to the posterior 

 commissure, and converging vmite with the main trunk. The main 

 trunk emerges into the ventricle a short distance anterior to the pos- 

 terior commissure. 



Within the brain tissue the main trunk of the fibre immediately 

 breaks up into fine divisions, whicli can be followed but a short distance 

 because of numerous neuroglia fibres which lie in their paths and ob- 

 scure the course of the individual fibres. A group of rather conspicuous 

 cells lies at the anterior end of the roof of the mesencephalon, i. e. the 

 torus, lateral to the median plane. Numerous fibres from this group of 

 cells run toward the ventricle where Reissner's fibre enters the brain. 

 As no direct connection has been seen, it is doubtful whether these cells 

 give rise to Reissner's fibre. 



Cyprinus niloticus. In specimens 8 cm. long Reissner's fibre has been 

 traced through its course to where it enters the brain. 



Salmonidae (64). In the Salmonidae I have studied the optic reflex 

 apparatus in three species, — the salmon (Salmo salar), the brook trout 

 (Salvelinus fontinalis), and the lake trout (Cristi vomer namaycush). 

 Between the first two there are no essential difi"erences, but the last shows 

 minor variations. In all, the apparatus is developed early. It is far ad- 

 vanced at the time of hatching, and becomes fully established and func- 



