SARGENT: THE OPTIC KEFLEX APPARATUS OF VERTEBRATES. 193 



of tlie cells of the nucleus inagnocellularis, he describes a tract of fibres, 

 Avhich were impregnated in his preparations, running from this region 

 to the cerebellum. This "tractus tecto-cerebellaris I " consists of coarse 

 fibres and "^ arises from tlie cephalic part of the tectum." " I have been 

 unable," he adds, " to find its cells of origin. The greater number of 

 the fibres arise from the vicinity of the large-celled nucleus. I have 

 traced the fibres down among the cells, but there are no cells impreg- 

 nated when the fibres are stained. Other fibres join this bundle from the 

 cephalic border of the decussation. The fibres, which are relatively 

 coarse and are beautifully impregnated in many preparations, form a 

 compact bundle which courses around the lateral border of the tectum 

 in the bottom of the shallow groove which limits the tectum laterally. 

 As it proceeds it is augmented by occasional fibres from tlie lateral part 

 of the tectum. When the bundle reaches the point of junction of the 

 tectum and cerebellum, it enters the cerebellum and is di.stributed to the 

 body and valvula. As the bundle enters the cerebellum the fibres groAV 

 distinctly larger, then divide into two nearly equal branches, each of 

 which further increases in size and becomes rough and irregular like a 

 dendrite. One branch of each fibre ends in the valvula, and these have 

 no regular arrangement. The other branch takes a straight course with 

 its fellows in a bundle along the lateral surface of the body, just ventral 

 to the line of connection with the lateral lobes. They break up in end 

 branches in the lateral portion ( granular layer) of the body, although 

 a few fibres reach the keel (molecular layer) at the caudal end of the 

 body." 



By the aid of Johnston's description I have been able to follow this 

 tract in my hematoxylin preparations of Amia, though not continuously. 

 It is, I believe, derived from the third process of the cells of the nucleus 

 magnocellularis, and is homologous with the tecto-cerebellar tract which 

 I have described in selachians and birds. It establishes connection 

 between the optic reflex apparatus and the cerebellum, by which the co- 

 ordination of muscular movements is brouglit about. 



The fact that the tract in Acipenser " courses around the lateral bor- 

 der of the tectum," while in Raja it passes straight back through the 

 roof of the tectum just lateral to the median plane, does not militate 

 against this homology, as the terminals are the same, and in each case 

 the shortest path is followed (see Johnston's Figure 4). Moreover, the 

 change in the relative position of this tract is easily understood in taking 

 into account the great lateral development of the optic lobes in Raja. 



Johnston has described in Acipenser certain small fusiform cells, 



