480 PEOF. G-. ELLIOT SMITH ON THE IMOKPHOLOGY OF THE 



This peculiar commissure has a very restricted distribution in the Vertebrate series. 

 It is not found in the Chelonia nor in the Crocodilia, but seems to occur in all the 

 Lacertilia (and perhaps in some Ophidia, although further investigations are needed to 

 harmonize the conflicting statements of Edinger, Herrick, and Rabl-Riickhard upon this 

 point *). There is no corresponding commissure in the Mammalia, nor is there any 

 exactly analogous arrangement elsewhere in the Vertebrate series. As the term 

 " commissura })allii posterior " i^ in many respects misleading and inapj)ropriate, as I 

 shall have occasion to point out later on, I have introduced the name commissura aberrcois 

 in reference to its peculiarly aberrant nature and limited distribution. 



In Dendy's otherwise excellent figure representing this commissure (fig. 15) the 

 latter has been erroneously represented as having a complete epithelial sheath. I 

 have represented its actual relation to the roof of the forebrain in fig. 2 (p. 457) 

 of tills memoir. 



No stronger testimony of the failure to understand the meaning of this commissure 

 coidd be adduced than the fact that Edinger, who has contributed a not insignificant 

 share to our knowledge of the commissura aherrans, should make such fundamental 

 errors regarding its position as I have mentioned above. But such statements are typical 

 of the obscurity which invests this peculiar feature of the Saurian brain ; and it becomes 

 necessary therefore, in consideration of the foregoing analytical paragraphs, to reconsider 

 the whole question of the possible significance of this commissure, because the literature 

 of the subject affords no sufficiently well-founded clue to its nature to be of any serious 

 value. 



In the preliminary study of the roof of the forebrain in Splienodon^ which served as an 

 introduction to this account, it was clearly demonstrated (^cide figs. 1 and 2) that tlie 

 commissura aberraiis had no connection with the lamina terminalis, but is placed in a 

 fold of the epithelial roof which is separated from ihe lamina terminalis not only by the 

 Imnina chorioidea (the fold from which the lateral choroid plexuses are derived) but also 

 by the pai'aphysis. The commissura aherrans, moreover, is situated caudad of the foramen 

 of Monro, and it marks (as I have already showni in the introduction) the line of 

 demarcation between the strictly thalamic portion of the roof and the more cephalic 

 region which is closely related to the cerebral hemisphere. It is therefore situated at a 

 most important morphological site, the exact location of which may be more readily 

 appreciated from a schematic plan of the forebrain such as the accompanying diagram 

 (fig. 21). In this scheme the mesial cavity or third ventricle will be seen to extend 

 forward as far as the lamina terminalis, and then to communicate by means of the 

 foramen of Monro on each side with the lateral ventricle. The lateral walls of the third 

 ventricle in this diagram are formed by the optic thalami, and the anterior extremity of 

 each thalamus is joined by an attenuated band to the tliiu free cortical fold which forms 

 he lahiiiDi. cundaJe of the cerebral hemisphere. This cortical fold is the hippocampus. 

 Erom it fornix-lihres arise and proceed towards the dorsal surface of the cephalic 



* Edinger auJ Eabl-Eiickhard are inclined to regard it as being absent in ISnakcs. I Lave been unable to find 

 am' trace of il in the Australian Suake Pseudechis. 



