THE BKAIN IN THE EDENTATA. 301 



smell is the main determinant, it is not surprising to find that the olfactory areas of 

 the In-ain reach a relatively larger development than in the arboreal Bradyjiodida' and 

 Cycloturiis, in \yliich the smell-sense is not of such vital importance. And in such 

 a form as Chlamydophorus, which lives mainly underground and depends to a very 

 slight degree upon its feeble visual organs, it is natural that the importance of the 

 olfactory ajiparatus should be still further enhanced, as finds expression in a relatively 

 enormous olfactory bulb and tuherculimi olf actor iitm, and a pyriform lobe which includes 

 more than half the lateral sui'face of the cerebral hemisphere. 



In a short memoir upon tbe basal regions of the hemisphere, Gustav Retzius last year 

 called attcjition to certain features of the pyriform lobe and the adjoining areas in some 

 Edentates, among other mammals *. He makes special mention of the little " tubercle 

 of the external olfactory tract " in Marsupials and Edentates. He dignifies this little 

 body Avith the name (jyrus iufermedms rluueiicephuli (p. 107), but although he carefully 

 describes its situation, he does not give us any information concerning its structiu'e. 



It will prove a source of considerable confusion if the term " gyrus " is applied to such 

 structures as tliis, involving as it does a considerable distortion of the generally accepted 

 idea of the meaning of this term. Retzius goes on to state that in the same animals 

 {Bidelphys, Ilacroptis, Myrmecoplmgu, Basypus among others) two or three sagittal 

 " gyri " are found behind the "gyrus intermedins." He calls these the "gyrus lunarls '* 

 and " gyj-usambiens {medialis and laterulis),'" in conformity with a nomenclature he had 

 previously apialied to the human brain f. 



He further mentions that in the Insectivora {Erinaceus) there Is a very large "gyrus 

 lunaris " and a " gyrus ambiens.'" 



Retzius does not make any mention of the fact that, in Erinaceus and many of the 

 Edentates, the fascia dentata (as we shall explain subsequently) extends on to the base 

 of the brain in the region of his " gyrus lunaris.'' Its relation to the latter structure 

 is well shown in the figure of the base of the brain in MyrmecopUaga (fig. 6). 



The Cerebral Commissures. 



In all vertebrates, series of nerve-fibres proceed from one cerebral hemisphere to the 

 other, and serve to bring into functional association brain areas which are otherwise 

 quite separate the one from the other. These fibres, whether they connect strictly 

 homologous areas or constitute a symmetrical bond between heterologous parts of the 

 two hemispheres, are generally known under the comprehensive title of " commissures." 



In all excej)t the lowliest vertebrates there are two compact and \Aell-defined bu.ndles 

 of these crossing fibres forming ventral and dorsal commissures of the cerebral hemisphere. 

 Within the class of mammals the constitution of these two commissures becomes rearranged 

 in order to more readily accommodate the enormously increasing number of fibres which 



* Gustav Eetzius, " Zur Kenntuis der Wiiiduugen der Riechhirns,"' Verhandl. Anat. Gesellsch. 1897, p. 10.5 

 et seq. 



t Gustav Eetzius, ' Das Menschenhirn,' 1896. 



SECOND series. — ZOOLOGY, VOL. VII. 42 



