December 2- 



[909] 



NA TURE 



217 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part o/ Nature. 

 No notice is taken of anonymous communications.] 



The Function of Reissner's Fibre and the Ependymal 

 Groove. 



Thanks mainly to the investigations of Porter E. 

 Sargent, Reissner's fibre is now known to occur through- 

 out the vertebrate series from the lamprey upwards. Not 

 long ago Mr. G. E. NichoUs directed attention in these 

 columns to its occurrence in the frog, and Sir Victor 

 Horsley {Brain, vol. xxxi., 190S) has recently shown that 

 it occurs in monkeys (Macacus). I have myself already 

 described it in Geotria, and have lately observed it in the 

 tuatara and in the cat. Its position and relations are re- 

 markably constant. Commencing beneath the posterior 

 commissure, it extends backwards to the hinder extremity of 

 the spinal cord, lying, at any rate for the greater part 

 of its length, quite free in the iter, the fourth ventricle, 

 and the canalis centralis. Anteriorly it breaks up into 

 very slender fUjrils, which are attached to the columnar 

 epithelium of the ependymal groove. 



The ependymal groove, though its existence has been 

 -trnngely ignored by most observers, also occurs through- 

 dut the vertebrate series from cyclostomes to mammals. 

 It is found typically as a deep longitudinal furrow on the 

 ventral aspect of the posterior commissure, lined by high 

 columnar epithelium, very different in appearance from 

 that which lines the greater part of the brain-cavity. In 

 this highly specialised condition I have observed it in 

 the lamprey, the tuatara, and the cat. I suggested 

 some years ago, in the case of the lamprey, before 

 the connection of this structure with Reissner's fibre was 

 known, that it might aid in the circulation of the cerebro- 

 spinal fluid, by means of cilia which I thought I had 

 detected on the epithelial cells. Whether this be so or not, 

 I now think that the ependymal groove may have another, 

 and perhaps more important function as an intra- 

 cerebral sense-organ. 



Sargent, as is well known, has interpreted Reissner's 

 fibre as a nervous structure which serves for the " short- 

 circuiting " of motor (optic) reflexes, and he regards the 

 epithelium of the ependymal groove merely as an attach- 

 ment plate for the fibre. This seems to me hardly a 

 sufficient explanation of the existence of such a highly 

 differentiated organ. 



The nervous nature of Reissner's fibre is very far from 

 having been demonstrated, and though I was formerly 

 led to agree with Sargent's opinion on this subject, 

 I can do so no longer. Sir Victor Horsley and Dr. 

 McNalty {loc. cit.) have shown that electrolytic lesions do 

 not cause any degenerative changes in Reissner's fibre such 

 as would be expected if it were a nerve-bundle, and this 

 result is, of course, totally opposed to that of Sargent's 

 earlier experiments, by which he endeavoured to est,ablish 

 the truth of the optic reflex theory. On the other hand, 

 there is a certain amount of evidence to show that 

 Reissner's fibre is highly elastic, and therefore more of 

 the nature of connective tissue. Thus it is often found 

 that when the spinal cord has been cut across, the fibre 

 has " sprung " and twisted itself into a knot or gnarl in 

 some part of the brain-cavity. This seems to indicate that 

 the fibre in life is under considerable tension. My 

 colleague, Mr. NichoUs, who has for some time past been 

 working at Reissner's fibre in this laboratory, will, I hope, 

 bring forward some further evidence on this head very 

 shortly. 



I now wish to suggest, for the consideration of physio- 

 logists, that Reissner's fibre and the epithelium of the 

 ependymal groove may form part of an apparatus for 

 regulating flexure of the body. Any such flexure would, 

 it appears to me, tend to alter the tension of Reissner's 

 fibre, and thereby exert a mechanical stimulus upon the 

 cells of the ependymal groove to which it is attached. We 

 NO. 2095, VOL. 82] 



may suppose that the stimulus received by these sensoiy 

 cells is transmitted to appropriate nerve-cells in the brain, 

 and that the deviations of the long axis of the body from 

 the normal position may possibly be regulated by reflex 

 action. We may compare the function of the semicircular 

 canals of vertebrates; also that of the " statocysts " of many 

 invertebrates, which serve, by means of mechanical stimu'i 

 due to the action of gravity, automatically to regulate the 

 orientation of the body. 



The position of Reissner's fibre, entirely enclosed within 

 the brain and spinal cord, renders it extremely difficult to 

 perform any experiments to test the truth of this hypo- 

 thesis, but I hope that the ingenuity of experimental 

 physiologists may overcome even such a formidable obstacle 

 as this. 



Arthur Dendy. 



Zoological Department, King's College, 

 Strand, W.C. 



With reference to Prof. Dendy 's remarks on Reissner's 

 fibre and the suggestion that he puts forward as to its 

 function, I may perhaps be permitted to add a few 

 words. 



Of the preformed nature and of the universal occurrence 

 of the fibre throughout the vertebrate series there can 

 remain, I think, no possible doubt. 



During the past two years in which I have been engaged 

 in investigating this structure I have examined sections of 

 the brains of a large number of specimens of more than 

 five-and-twenty different genera taken at random from all 

 the great classes of vertebrates, and in no case have I 

 failed to find the fibre in properly fixed material. Any of 

 the commonly employed fixing reagents may be used, but 

 it is essential that the material shall be fixed immediately 

 after death. 



I have been greatly impressed with the extraordinary 

 eiasticity of the fibre, which in life appears normally to 

 be under considerable tension, so that if it be severed in 



Photomicrograph of a section of the terminal portion of ih; sf inal 

 cord of the lamprey. x6oo. 



the fresh state the free ends recoil sharply. In one case, 

 where in removing the brain of a toad the fibre was 

 accidentally snapped, so considerable was this recoil that, 

 as subsequent e.xamination of the sections revealed, the 

 free end of the fibre sprang forward into the cavity 

 of the fore-brain, the thin roof of whicli it actually per- 

 forated. Another striking instance of this same character 

 is furnished in certain sections of the terminal portion of 

 the spinal cord of the lamprey, a photomicrograph of 

 which, kindly taken for me by Mr. R. W. H. Row, is 

 here reproduced. 



In this case the cord was severed when fixation was only 

 partially effected, and a sudden recoil having been pre- 

 %'ented by the partial fixation, the fibre was withdrawn 

 backwards towards its attached end by a more gradual 

 retraction that resulted in the production of a number 

 (twenty-nine) of close coils, two of which appear in the 

 photograph. 



From these and other facts I am persuaded that the 

 structure is not a nerve, but rather an elastic rod with a 

 merely mechanical function, and to my mind the sugges- 



