322 DK. G. ELLIOT SMITH ON 



we find tliat the structure is quite as atrophic as the relatively much more insignificant 

 vestiges of the hippocanipus in the more highly-organized mammals. 



There is another reason of quite different natui'e for entering so minutely into the 

 anatomy of this region in the Edentata. The clear demonstration of the hippocampal 

 region which the Edentate brain affoi'ds rendei's it a particularly suitable object upon 

 which to demonstrate the typical features of a region which has given rise to groattr 

 perplexity, perhaps, than any other of the brain. 



Eor u.any years the question of the mode of ending of the dorsal extremity of the 

 liippocampus has excited a great deal of controversy. Several investigators have from 

 time to time given an acciu-ate account of its behaviour in different mammals. Thus 

 Henle and Giacomini long ago demonstrated that the hippocampal formation extends 

 on to the dorsal aspect of the spliMiium in Man. Other writers, on the other hand, have 

 emphatically denied this and affirmed that the hippocampus is always subcallosal. 

 Other writers, again (such as Zuckerkandl, who has a large following), have stated that 

 the fascia dentata extends on to the upper surface of the corpus callosum. Then, again, 

 Ganser says that in the Mole the fascia dentata ceases upon the ventral aspect of the 

 corpus callosum, but that the layer of pyramidal cells {i. e. the hippocampus proper) 

 becomes continuous with the rudimentary supracallosal ijidusium. We have, then, 

 in the literature of this subject four mutually contradictory beliefs concerning the 

 mode of termination of the upper extremity of the hippocampus, each maintained by its 

 own supporters with much cogency. 



We have already seen in the different Edentates that the manner in which the hippo- 

 campus terminates can be clearly demonstrated, and we can also see with the naked eye 

 the cephalic extension of the hippocampal vestige, not only to the anterior extremity cf 

 the corpus callosum, but also as far as the olfactory peduncle. No other writer upon 

 this subject seems to have even suspected the presence of this cephalic extension of the 

 hippocampus. Eor it has been possible, upon the evidence of comparative data, to 

 definitely state that the indus'mm is not only continuous with the hippocainpus, but in 

 itself atrophied hippocampus *. 



" The Pabacommissueal Body." 



In examining the mesial surface of the hemisphere we find, in front of the psalterium 

 and ventral commissure, a region to which passing reference has been already made as 

 the area piYrcommissitralis. This term was originally introduced by the writer as a 

 pui-ely descriptive title for a region of doubtful identity in front of the cerebral 

 commissures in Ornithorhynclms. This was subsequently shown to he the surface of a 

 large ganglionic mass to which the name corpus prcecommissurale was given t. In the 

 course of further investigations which were carried on after the introduction of the 



* For the evidence of this point see ' Journal of Anatomy and Phj'aiology,' irp, cit. vol. xxxii. 

 t Cf. " The Origin of the Corpus Callosum," this volume, p. 47. 



