SARGENT : THE OPTIC KEFLEX APPARATUS OF VERTEBRATES. 177 



the fibre of Reissner. Those cells of the roof-uucleus lying in the poste- 

 rior region have a different termination for their axons from the one 

 just described. In this instance the axons pass posteriorly, instead of 

 anteriorly, and they take a course into the cerebellum." As I have 

 before stated, I believe that each cell sends an axon anteriorly and also 

 a cei'ebellar neurite posteriorly into the cerebellum. 



The tractus tecto-cerebellaris has been observed in selachians by San- 

 ders ('86, p. 749) and Haller ('98, p. 514), but they both failed to rec- 

 ognize its connection with the ' Dachkern.' Its origin was discovered 

 independently and perhaps simultaneously by Edinger (:0l) and Sargent 

 (:01''). Of its termination in the cerebellum, Edinger says merely: 

 "Der ganze Zug, der tlrbrigens aus nur relativ sehr dicken Fasern 

 besteht, verschwindet im Velum anticum caudad von der Trochlearis- 

 wurzelkreuzung." 



In one other particular Houser's observations differ from mine, — as to 

 the nature of the connection between the cells, and the optic fibres. 

 Houser has described " nerve-fibres emerging from the stratum medul- 

 lare profuudum and terminating in arborizations near the bodies of the 

 nerve-cells." He fails to mention any other mode of communication. 

 Although I have seen and described (p. 167) such fibres, terminating in 

 arborizations over the bodies of the cells, and think it quite probable 

 that they come from the stratum meduUare profundum, I believe the 

 more essential connection between the two is by the dendrites arising 

 from the cells of the 'Dachkern' and terminating in the stratum 

 medullare profundum. 



Houser (p. 139) evidently believes that the olfactory centres are con- 

 nected with Reissner's fibre. '* The neurones of the nucleus strati grisei 

 are, primarily, a relay in the olfacto-motor chain. . . . The axones 

 from the cells of the nucleus strati grisei pass backwards into the base 

 of the midbrain as the tractus thalamo-tectalis, and then sweep upward 

 into the tectum to lie in the stratum medullare profundum. Here they 

 are associated with other sensory nerve-fibres, . . . and the entire group 

 becomes related to the remarkable motor-conducting path provided by 

 the cells of the roof-nucleus and the fibre of Eeissner." But his evi- 

 dence for this is not given in detail. I have recorded observations else- 

 where in this paper which lead to the belief that there is a connection 

 of the olfactory centres with the fibre of Eeissner, but in a very different 

 way from that suggested by Houser. 



