218 HOVEY JORDAN 



He regarded it as being connected at its anterior end chiefly 

 with the optic centers of the brain, at its posterior end with the 

 motor roots of the spinal cord, and, farthest back, with certain 

 so-called 'posterior canal cells,' which are themselves connected 

 to the walls of the cord. In several papers, written either con- 

 jointly or individually, Dendy and Nicholls (1902 to 1917), on 

 the contrary, have maintained that the fiber itself is non- 

 nervous; that functionally it is a mechanical apparatus for 

 controlling "automatically the flexure and pose of the body." 

 It is thought to do this by exerting a tension upon the ven- 

 tricular fibers of certain supposedly sensory cells in the region 

 of the posterior commissure of the brain and in the spinal cord, 

 with which the fiber is believed to be connected. 



My own studies have been made chiefly upon two teleosts, 

 the brook-trout (Salvelinus fontinalis) and the hamlet (Epineph- 

 elus striatus). I am of the opinion that Reissner's fiber is 

 ependymal in origin and that it does not function as a nervous 

 apparatus either directly, as maintained by Sargent, or indi- 

 rectly, as argued by Dendy and Nicholls. 



In sections of young trout brains, which were purposely cut in 

 a parasagittal plane (figs. 1, 2, A, and B), the fiber is seen to 

 originate (as shown by Sargent) from the region surrounding the 

 posterior commissure. In the trout, however, its cells of origin 

 are all confined (as shown by Nicholls for other forms) to that 

 median strip of columnar epithelium which extends along the 

 anterior face of the region of the posterior commissure from its 

 ventral border nearly to its dorsal extremity, the 'sub-com- 

 missural organ' of Dendy and Nicholls. From the superficial 

 end of nearly every cell, probably from all of them, a very fine 

 fibril ifhii.'), which is apparently single, and in the epithelium 

 lies within a sheath, extends, without the sheath, into the lumen 

 of the brain and fuses with those from other cells into somewhat 

 larger fibrillae. These, in turn, converge to a point a little back 

 of the posterior commissure (figs. 1, 2, and B) to constitute the 

 extreme anterior end of Reissner's fiber. These fibers when cut 

 tran'^versely (figs. 2, 3, fhrl.") appear as small dots, each sur- 

 rounded by a circle — the cross-section of the sheath. 



