203 



Ornithorhynchus has a trigeminal nerve relatively larger than that 

 of any other mammal, as I have pointed out in previous communi- 

 cations i), and this nerve is distributed to the highly sensitive and 

 exceedingly delicate skin of the extensive snout, which is supplied with 

 peculiarly modified special organs of touch [described in detail by 

 Wilson and Martin '^)]. No mammal therefore can have a more highly 

 developed "oral sense" than Ornithorhynchus, and, as the olfactory 

 apparatus is very poorly developed in this animal, the contrast between 

 the two rival claimants to representation in the tuberculum olfactorium 

 is still further emphasized. As a further control upon any inference 

 we may draw from the conditions found in Ornithorhynchus there is 

 the evidence afforded by the nearly related animal. Echidna, with a 

 much more insignificant trigeminal nerve supplying a smaller and less 

 sensitive beak and a highly developed olfactory apparatus. If Edinger's 

 hypothesis were true we ought to find an enormous tuberculum ol- 

 factorium in Ornithorhynchus and a relatively diminutive one in 

 Echidna. But exactly the reverse is the case. Ornithorhynchus has a 

 particularly small tuberculum olfactorium, the cortex of which is in an 

 atrophic condition, whereas Echidna has a much larger tuberculum 

 with a well-developed cortex^). 



In the anosmatic Cetacea the cortex of the tuberculum olfactorium 

 atrophies, as I have pointed out on several occasions'^), although the 

 trigeminal nerve is well-developed. 



It is clear, therefore, that the size of the tuberculum olfactorium 

 cannot be determined by the presence of a sensitive snout and a large 

 trigeminal nerve. 



I now propose to call attention to more positive and conclusive 

 evidence of the intimate connection of the tuberculum olfactorium and 

 the bulbus olfactorius. 



Fourteen years ago while examining with a lens the fresh brain 

 of a Perameles I noticed a series of fine pencils of nerve-fibres leaving 

 the olfactory tract (in the groove between the pyriform lobe and the 

 tuberculum olfactorium) and passing on to the surface of the olfactory 

 tubercle, apparently to end there. In a number of drawings, which 

 I made at that time, these fibres are represented; but neither these 

 sketches nor the results of the extirpations, to which I shall refer 



1) Anatomy of the Brain in the Monotremata. Journ. of Anat. and 

 Physiol., Vol. 33, 1899. 



2) Anatomy of the Muzzle of Ornithorhynchus. Proceedings of the 

 Linnean Society of New South Wales, 2. Ser., Vol. 9, 1894, p. 660—681. 



3) op. cit., Journ. of Anat. and Physiol., 1899, p. 337. 



4) op. cit, p. 338. 



