INVESTIGATION ON THE FUNCTION OF REISSNER's FIBRE. 567 



commissure. This structure, for which the name sub-commissural 

 organ has been proposed (and wliicli, as I shall hope to show in a paper 

 now nearly ready for publication, develops from an anlage in tlie 

 brain, which is serially homologous with the anlage of the lateral and 

 pineal eyes), must he looked upon as an intra-cerehral sense organ. In 

 early development a paired structure, it takes up, in many forms, a 

 median dorsal position, and in almost all vertebrates becomes in the 

 adult a most conspicuous structure in the mid-brain. 



It is from the internal (ventricular) aspect of the cells of this organ 

 that Eeissner's fibre arises as a large number of cilia-like fibrillae, 

 which converge beneath and behind the posterior commissure into a 

 rod-like structure which may, at its anterior end, be either paired at 

 first or single and median. In either case it shortly becomes a single 

 median thread and stretches backward as such to the extreme hind end 

 of the central nervous system. Beneath the rhombo-mesencephalic 

 fold it frequently comes to lie in a well-marked dorsal median groove 

 (the " isthmic canal "), which deepens with age, and which may be 

 paired if the paired character of the fibre is maintained so far 

 caudally. 



Through the central canal of the spinal cord Eeissner's fibre may be 

 traced backwards lying centrally and apparently supported at frequent 

 intervals by cilia from the ependymal cells. 



At the actual extremity of the spinal cord (filum terminale) the 

 central canal widens out into a sub-spherical space which was named 

 by Ketzius* the sinus (ventriculus) terminalis. This chamber is not, 

 however, wholly enclosed within the nervous system, for, posteriorly, 

 the ependymal epithelium — which alone constitutes this part of the 

 filum terminale — fails entirely, and there is left a wide opening which 

 I have called the " terminal neural pore." The wall of the sinus 

 terminalis is thus completed posteriorly only by the connective tissue 

 sheath of the spinal cord. Into this meningeal wall Eeissner's fibre, 

 flaring out into a trumpet-like end, passes and is inserted. 



If in freshly killed material Eeissner's fibre be cut, it recoils spirally 

 in both directions from the point of section, forming dense tangled 

 knots such as would be formed in a thin elastic thread which, held 

 firmly at one end, ivus twisfxd from the free end continually in one direction. 

 In this reaction, as also in its straining reactions, in its origin in the 

 brain, and in its ending in the meninges, Eeissner's fibre is altogether 

 unlike any known nerve. 



Since then it is not a nerve, Sargent's " Optic Eetiex Theory " can no 



* Retzius knew of its occurrence only in Amphioxus and Cyclostomes. It is to be found 

 even better developed in Elasmobranchs and Teleosts. 



