1910.] On a Mesoccelic Recess in the Human Brain, etc. 527 



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 foramen 

 in the neural tube with tlie surrounding connective tissue. Thus Studnicka's 

 statements on the subject (1899), controverted by Sargent (1904), 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, 1909) 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 Eeissner'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 tension of Reissner's fibre, and 

 thereby exert a mechanical stimulus upon the epithelial cells of the sub- 

 commissural organ to which it is attached anteriorly. It was supposed 

 that the stimulus thus received by the sensory epithelial cells might be 

 transmitted to appropriate nerve-cells in the brain, and that the deviations 

 of the long axis of the body from the normal position might thus be 

 regulated by reflex action ; and a comparison was made with the function 

 of the semicircular canals of vertebrates, and also with that of the 

 " Statocysts " of many invertebrates, which serve by means of mechanical 

 stimuh, due in this case to the action of gravity, automatically to regulate 

 the orientation of the body. 



The fact that man, almost the only vertebrate which has assumed the 

 erect posture, and one of the few which have completely lost the tail, also 

 has the sub-commissural organ in a more reduced condition than in any 

 other case known, while the semi-erect and almost tailless chimpanzee is in 

 this respect in an intermediate condition between man and the lower 

 vertebrates, seems clearly to indicate that the function of the sub-commissural 

 organ is in some way connected with the position of the body and the 

 flexibility of the vertebral column (including of course the tail). 



Considering that the sub-commissural organ, with which Reissner's fibre, 

 when present, is invariably connected, is reduced to a mere vestige in the 



