494 



PROF. G. ELLIOT SMITH OX THE :\rOEPHOLOGT OF THE 



campus at a time before the attemiation of the hippocampo-thalamic band has begim, 

 and when, therefore, there is a close connection between not only the hippocampus and 

 the thalamus, but also the roof of the forebraiu and both of these structures. 



That commissural fibres of the hippocampus should avail themselves of this direct 

 path across the roof of the forebrain is what one would naturally expect. The puzzling 

 feature of the problem is that the corresponding fibres in the Chclonia and Crocodilia 

 do not make use of this route, but pursue the more circuitous path via the dorsal 

 commissure. 



Fig. 30. 



O.C. 



Sketch of the dorsal aspect of the left half of the forebrain oi u human foetus, drawn from His"s tig. 7, Taf. i. {Joe. 

 eit.), with certain moditications suggested by Hochstetters researches (' Zur Entwick. d. Gehirns,' 18!)8). 



Eoof of cerebral hemisphere dissected away. 



Summing up the conclusions of the foregoing arguments, it seems that the commisstira 

 (iberrans is a bundle of fibres derived from the caiulal portion of the hippocampus, and 

 therefore homologous (in part) with the psalterium of the Mammalia. But its behaviour 

 presents a marked contrast to that of the Mammalia : for, instead of pursuing an 

 extensive forward course to cross over in the lamina terminaUs, it avails itself of the 

 primitive direct connection between the caudal lip of the cerebral hemisphere and the 

 optic thalamus, and in this way reaches the roof of the third ventricle dLrectly. This com- 

 missure is found only in Sphenodon and the Lacertilia (and possibly in certain Ophidia) 

 as a separate bundle. From examination of the brain in a number of Amphibia and 

 related forms, it seems certain that an analogous commissui'c is found there also, but 

 that instead of pursuing an independent coiu-sc it crosses the roof of the third ventricle 

 alongside the fibres of the commisstira superior of Osborn. This phenomenon is readily 

 understood when it is remembered that the thalamic region is very short in the 

 Ichthyopsida, and the sitiiations of the commissnra uberrans and coramissura s/qjer/or 

 would be closely approximated. The fact that the so-called commissuro superioi- of 

 Amphibia derives fibres from the caudal extremity of tlie cerebral hemisphere is well 

 known, and has been recorded by almost all writers upon the Amphibian brain, so that 

 it is unnecessary for me to adduce evidence to prove this matter of common knowledge. 



