SARGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 155 



median plane. Toward the median end of their course they run parallel 

 with the fibres of the posterior commissure, from which, however, they 

 are readily distinguished by their greater diameter and greater sharp- 

 ness of outline, and by the absence of medullary sheaths. In the latter 

 part of their course they are usually collected into small fascicles of 

 from 3 to 12 fibres. On reaching a position dorsal to the ventricular 

 grooves before described, they bend rather sharply at approximately a 

 right angle, and passing between the ependymal cells enter the meso- 

 cephalic recess, where they join the trunks of Reissner's fibre (Figs. 

 6, /as. Rei)^., and 7). Throughout the length of these lateral grooves 

 in both transverse and sagittal sections, axons may occasionally be seen 

 penetrating into the ventricle radially from between the ependymal 

 cells, and joining the axis-cylinder-like trunks of Reissner's fibre. The 

 cells from which these fibres arise have not always been recognized, but 

 they are probably tectal reflex cells of inconspicuous size, which lie in 

 the lateral portion of the tectum opticuni. 



c. Hahejmlar Constituents of Reiss7ier\f Fihre. In Petromyzon there 

 are still other constituents of Reissner's fibre. At the anterior ex- 

 tremity of the right lateral ependymal groove, at the base of the right 

 ganglion habenula the axons of another group penetrate between the 

 cells of the ependymal thickening and enter the groove in the roof of 

 the diacoele (VXvde. 1, Y\g. b, fhr. hal). Reis.). Large elongated multi- 

 polar cells, lying at the base of the ganglion close to tlie ependymal 

 thickening bordering the groove, furnish the greater number of these 

 axons. Some of their neurites run toward the dorsal part of the 

 ganglion. 



A well-marked tract, consisting of a small number of sharply defined 

 fibres, runs from the right ganglion habenula to the anterior end of the 

 right lateral groove. This tract has its cells of origin located in the 

 deeper parts of the ganglion. It seems to contribute some of the axons 

 which penetrate into the diacoele. 



The left lateral groove in the roof of tlie diacoele with its ependymal 

 lining is not so well developed as that of the right side, corresponding 

 in this with the dwarfed condition of the left ganglion habenula. I 

 have been unable to distinguish any axons entering the diacoele from 

 the left ganglion, though there may be a small number which have 

 escaped my notice. 



The axons entering the diacoele collect into a single trunk, which 

 runs caudad into the niesocoele and at a point caudad of the posterior 

 commissure unites with the trunk from the ri<iht horn <>f the recessus. 



