506 



the adult. We have, however, only been able to examine one series 

 of sections in each case. Probably, like many other vestigial structures, 

 it will be found to vary considerably in the degree to which it per- 

 sists in diiferent individuals. 



IV. The Function of the Sub-Commissural Organ and 



Reissner's Fibre. 



Sargent, who has done so much to extend our knowledge of 

 Reissner's fibre, considered (1904) this structure to be of a nervous 

 nature, forming a kind of short circuit for optic motor reflexes. Our 

 own histological investigations on many types do not support this view, 

 nor do the physiological experiments of Sir Victor Horsley and Dr. 

 McNalty (1908). On the other hand, there is every reason to be- 

 lieve that Reissner's fibre is a highly elastic structure, for it is well 

 known that it has a strong tendency, when cut across, to spring back 

 and coil itself into knots and spirals ^). Moreover, one of us (Nicholls) 

 has been able recently to demonstrate beyond question that at its 

 posterior end (in the lamprey and various teleosts) the fibre is not 

 connected with ''canal cells" in the canalis centralis of the spinal cord, 

 as maintained by Sargent, but is connected through a terminal fo- 

 ramen in the neural tube with the surrounding connective tissue. Thus 

 Studnicka's statements on the subject (1899), controverted by Sar- 

 gent (1903), are confirmed. At its anterior end the fibre appears 

 always to break up into very slender branches, which are connected 

 with the epithelial cells of the sub-commissural organ. 



Reissner's fibre and the sub-commissural organ occur throughout 

 the vertebrate series from the cyclostomes to the primates, and there 

 can be little doubt that where they are (as in nearly all cases) well 

 developed, they must have some important function. 



In a letter recently published in "Nature" one of us (Dendy, 1908) 

 has made a suggestion as to the possible function of these organs, 

 which appears to us to be strongly supported by the results recorded 

 in the present communication. This suggestion was to the effect that 

 Reissner's fibre and the sub-commissural organ ("Ependymal Groove") 

 may form part of an apparatus for regulating flexure of the body, 

 It was pointed out that any such flexure would tend to alter the ten- 

 sion of Reissner's fibre, and thereby exert a mechanical stimulus 

 upon the epithelial cells of the sub-commissural organ to which it is 



1) Cf. Nicholls (1909). 



