51 



a prominent (in lower Mammals) hemispherical grey mass on the mesial 

 side of the anterior part of the pyriform lobe and (in a surface view 

 of the base) immediately behind the olfactory bulb, with which it is 

 connected by the short olfactory peduncle. It is the "Rinde am Streifen- 

 hügelkopfe" of Ganser, the "espace quadrilatere" of Broca, the "post- 

 rhinal lobe" of Herrick and the "orbital lobe" of Turner (in Dasy 

 pus sexcinctus). It also roughly corresponds to Edinger's "Riech- 

 feld", to the "locus perforatus anticus" of Vicq d' Aztr (in man) and 

 possibly to the "lobus post-olfactorius" of Burckhardt inProtopterus. 



It is covered superficially by a thin layer of medullated nerves 

 derived from the external olfactory radiation. Posteriorly it is con- 

 tinuous with the grey floor of the thalamencephalon and mesially with 

 the "precommissural area" (p. a.). Dorsally it is continuous with the 

 corpus striatum. Like the pyriform lobe, it varies in size with the 

 development of the olfactory bulb. Both areas probably reach their 

 maximum relative sizes for a Mammal in the small marsupial mole 

 — Notoryctes typhlops, whose cerebrum I have described in the 

 Transactions of the Royal Society of South Australia. 



These three parts of the brain — hippocampus, pyriform and 

 tuberculum olfactorium — together with the olfactory bulb, precom- 

 missural area and "septum lucidum" constitute the smell centre. That 

 some of these parts subserve other senses than that of smell is prob- 

 able, but that they are all more intimately connected with the ol- 

 factory than with any other sense-apparatus is equally certain. 



All the rest of the cortex may be distinguished as "pallium", 

 following Turner's nomenclature. It is an area of very variable ex- 

 tent, whose size does not appear to be directly related to that of any 

 sensory apparatus. Turner has shown that its relative size may be- 

 come greatly reduced in certain Mammals such as Erinaceus and 

 Talpa. In Perameles nasuta it is still further reduced, so that 

 it merely forms a small cap resting upon the rhinencephalon. In 

 Notoryctes it is still smaller. If this process of reduction were 

 continued any further, the result would be a brain practically with- 

 out "pallium". In lowly macrosmatic Mammals e. g. Talpa, Pera- 

 meles, the pyriform lobe extends round the ventro-lateral border to 

 constitute a large part of the lateral aspect of the cerebrum. Among 

 the Marsupials as one examines the brains ofMacropus, Dasyurus, 

 Phalangista, Koala, Perameles in turn, it will be found that 

 the hippocampus takes a greater share in the constitution of the median 

 hemisphere wall as the series is descended, until in Notoryctes 

 there is more hippocampus than median "pallium". 



