526 



Prof. A. Dendy and Mr. G. E. NichoUs. [May 24, 



In successive sections this irregular cavity can be traced forwards and 

 upwards, behind and above the posterior commissure. Tt obviously corre- 

 sponds to the similarly situated mesocoeiic recess in the foetus, although 

 lacking its characteristic high columnar ependymal lining. Anteriorly it 

 again becomes choked with scattered cells, forming an ill-defined tract which 

 passes forwards and then abruptly gives place to a small, but very well 

 defined, sub-spherical chamber (Plate 14, fig. 3, m.r.). This chamber, 

 moreover, is lined by exactly the same type of elongated epithelium as 

 characterises the sub-commissural organ in the foetus and in lower forms. 

 The photomicrograph reproduced in Plate 14, fig. 3, shows this chamber, 

 which we regard as simply the blind anterior termination of the mesoccelic 

 recess (compare Plate 14, fig. 1, and text-fig. 9). The lumen here has a 

 diameter of about 0*05 mm., while the cells of the lining epithelium are about 

 0'02 mm. long, with the nuclei placed as usual towards their inner ends. In 

 the lumen is a small quantity of coagulum. 



In tlie brain tissue around the posterior portion of the mesocoeiic recess 

 {i.e. that portion where the cavity Tias become irregular and largely 

 obliterated) occur numerous small globules (Plate 14, fig. 2, gl.) which have 

 stained brightly red in tlie preparations (which were treated with borax 

 carmine in bulk, followed, upon the slide, by picro-indigo-carmine). Whether 

 these are due to pathological or to post-mortem changes we are unable to say ; 

 that they are normally present seems improbable — in any case they are 

 conspicuous in these sections, and interesting as occurring closely adjacent to 

 the last vestiges of the sub-commissural organ. 



We have thus found the sub-commissural organ fairly well developed in 

 the human foetus, and in an entirely vestigial condition in tlie 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 persists in different individuals. 



IV. The Function of the Suh-Commissural Organ and Beissner's Fibre. 



Sargent, who lias done so much to extend our knowledge of Keissner's 

 fibre, considered (1904) this structure to be of a nervous nature, forming a 

 kind of short circuit for optic motor reflexes. Our own liistological 

 investigations on many types do not support this view, nor do tlie physio- 

 logical experiments of Sir Victor Ilorsley and Dr. McNalty (1908). On 

 the otlier hand, there is every reason to believe that Ileissner's fibre is a 

 liigliiy elastic structure, for it is well known that it has a strong tendency, 

 wiien cut across, to spring back and coil itself into knots and spirals.* 



* Cf. Nichollu (1909). 



