PARIETAL SENSE-ORGANS OF GEOTRIA. 17 



These fibrils are so extremely slender that it is almost too 

 much to hope for to find unbroken continuity, especially 

 when we remember that the coagulation of the fluid in the 

 brain-cavity and the shrinkage in preparation must tend to 

 cause rupture. In fig. 6 a great tuft of extremely fine 

 branches [R.F.') is shown coming off from Reissner's fibre 

 beneath the hinder part of the posterior commissure, whilst 

 more anteriorly the main fibre divides into two approximately 

 equal branches {B.F."), and at R.F.'" delicate fibrils are seen 

 emerging from the ependymal epithelium. These appearances 

 strongly confirm the observations of Sargent as to the con- 

 nection of Eeissner's fibre with the ependymal groove. As 

 to the origin of the constituent fibrils from optic reflex cells 

 within the substance of the brain, however, I am not able to 

 make any definite statement. In fig. 3 I have shown the 

 existence of a group of large nerve-cells {N.C.) situated in 

 the anterior lateral part of the tectum opticum on either 

 side of the posterior commissure. These obviously corre- 

 spond to two of the groups of optic reflex cells described 

 by Sargent in Petromyzon, and represented in his fig. 7, 

 but I have not seen any connection of these cells with the 

 ependymal groove, such as he figures. This, however, by 

 no means proves that no such connection exists, and it must 

 be remembered that my material was not specially prepared 

 for the purpose of tracing nerve fibres. 



I have, however, already shown that fibres from the pineal 

 nerve are connected with the ependymal epithelium on its 

 inner aspect, while Sargent has shown that branches of 

 Reissner's fibre are connected with the same epithelium on its 

 outer aspect. One is tempted to conclude, therefore, that the 

 pineal eye is connected with Reissner's fibre through the pineal 

 nerve, and thus linked up with the optic reflex system. This 

 conclusion obviously involves one in what appears at first 

 sight to be a very serious difficulty. It must be remembered 

 that the pineal nerve is apparently a sensory nerve, while 

 Reissner's fibre is a motor nerve, and a direct connection 

 between the two, without the intervention of ganglion cells in 



VOL. 51, PART 1 . — NEW SERIES 2 



