SARGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 177 



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

 rior re<>ion 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 cerebellar 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 

 (iOl"). Of its termination in the cerebellum, Edinger says merely: 

 "Der o-auze Ziv' der tirbrisens aus nur relativ sehr dicken Fasern 

 besteht, verschwindet im Velum antic um 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 profundum 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-raotor 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 Reissner." 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 Reissner, but in a very different 

 way from that suggested by Houser. 



