484 PEOF. G. ELLIOT SMITH ON THE MOEPHOLOGT OF THE 



TJie Com2Jarative Morphology of the Commissura aherrans. 



The problem of the interpretation of the cerebral commissures is, however, by no 

 means so simple as the above account may lead one to suppose. For there is iu 

 certain Reptiles the additional commissure, which I have distingviished by the name 

 " aberrans.'' So far as I am aware, E-abl-Riickhard * was the first writer to call attention 

 to it, in the brain of Pscmimqsaurns. He regarded it as a rudiment of the fornix or 

 " eine Kommissur fiir die Ammonshorner (Forel)." His reason for this was the fact 

 that its fibres spread out in the ventro-caudal region of the mesial wall of the hemisphere, 

 which he regarded as hippocampus, presumably because it occupies a position which is 

 topograi^hically analogous to that of the Eutherian hippocampus. Osborn wrongly 

 condemned this interpretation, and confused the commissura aherrans of the Reptilia 

 with the commisstira habenularum in the Frog, to which he had recently given the name 

 " coimuissuru superior' i. It is, however, only just to add tliat Osborn made this 

 comment before he had examined any brain 2:)ossessiug the aberrant commissure %. 



According to BrUl §, the first observation of the aberrant commissure must be 

 attributed to Spitzka, who recorded its presence in Iguana and called it '• corpus 

 callosum " the year before E,abl-E;uckliard described it in Psammosaiirus. 



Edinger confirmed Rabl-Eiickhard's observation, and adopted his suggestion concerning 

 its homology with the "■ commisura fornicis'' \. In this memoir he calls the commissura 

 dorsaUs by the name " corpus callosum," although, like the aberrant commissure, it 

 springs from the area which he correctly labels " Ammonsliorn.'' 



Two years later, Honegger ^ objected to the suggested liomology of the commissura. 

 aherrans with the fornix, and, if I correctly understand his somewhat involved argument, 

 he regarded it as the representative of the commissural fibres of the taenia semicircularis 

 (vel teriuinalis). 



* Kabl-lliickbard, " Uebcr (las Vorkommen eiues Fornixrudimeuts bei Eeptilien," Zoolog. Anz. 1881, p. 281. 



t H. F. Osborn, "The Origin of the Corpus CaUosum," Morph. Jahrb., Bd. xii. ]>. 243. 



+ [En pf'ssant I must refer to a matter not altogether pertinent to the sul)jeot of discussion. I do So in order to 

 prevent a confusion which constantly occurs in the literature of this part of the brain. Since the introduction of the 

 term " commissura superior" b}' Osborn for the habenular commissure, it has attained a very wide and general 

 recognition among comparative anatomists. The names of Osborn, Bellonci, Burckhardt, Shijaley, Edinger, Hitter, 

 Studnicka, Prenant, and Sorenson, to mention only a few of those who have employed this term, indicate how 

 general and cosmopolitan the use of this term is. And yet, since this almost universal adoption of the term, a 

 prolific writer on comparative neurology, who claims to be familiar with the literature relating to the reptilian 

 brain, applies this name to the dorsal or hippocampal commissure in the Monotreraata and ilarsupialia, viz., 

 Theodor Ziehen, in Jenaisch, Denkschr., 1897. 



While referring to a case (that of Osborii) in which the commissura superior had been mistaken for part of the 

 hippocampal commissure (c. aherrans), I wish to protest against the singular inappropriateness of applying this same 

 name "commissura superior" to tlie hippocampal commissure itself, and thereby deliberately introducing an elemeut 

 of confusion.] 



§ N. E. Brill, 'Medical Record,' March 29, ISiio, pp. 343-345. 



II Ludwig Edinger, op. cit. su^jra, 1888. 



% J. J. Honegger, " Uebcr den Fornix, &o.,'' Reeueil Zool. Suisse, tom. v., 1890. 



