518 



hemisphere — the trigoniim olfactorium of Human Anatomy — with 

 the tuberculum olfactorium, the peculiar cortical area which forms a 

 cap over the ventral surface of the corpus striatum ^). Thus it often 

 happens that writers apply the name tuberculum olfactorium to the 

 central attachment of the olfactory peduncle. Burckhardt seems 

 to have intended such an application of the term, when he employed 

 it in reference to the brain of Protopterus. [His recent collaborator, 

 Bing, says, indeed, that the term was used with this definite meaning,] 

 But the apparent olfactory peduncle is really the olfactory nerve; in 

 other words the olfactory bulb is not placed peripherally near the 

 nasal sac (as is the case in many Elasmobranchii), but is sessile on 

 the cerebral hemisphere; in other words, it forms a part of the wall 

 of the lateral ventricle. So that what Burckhardt seems to have 

 imagined to be the swollen central end of the pedunculus and called 

 "tuberculum olfactorium" is in reality the bulbus olfactorius itself. 



This curious mistake — the failure to recognise the olfactory 

 bulb and the confusion to which this gives rise — is repeated and 

 emphasized in all the references to Protopterus in the monograph on 

 Ceratodus by Bing and Burckhardt (op, cit. infra, p. 527), 



This is a most unfortunate mistake to have made, because the 

 true tuberculum olfactorium is present in a very highly developed and 

 peculiarly modified form in the brains of Protopterus and Lepidosiren. 

 It constitutes almost half the cerebral hemisphere and hangs down 

 on its ventral aspect as a large horizontally-placed sausage-shaped 

 bag — the "Lobus postolfactorius" of Burckhardt and his followers — , 

 which is such a characteristic feature of the brain in the Dipnoi and 

 especially the Dipneumona (see Figures 1, 8, 8', and 9 A, and 



1) In a recent number of the Journal of Anatomy and Physiology. 

 (Vol, 41, p, 251) I have published a diagram to explain these distinctions, 

 the failure to understand which is such a frequent source of confusion. 



At the time when Burckhardt wrote his memoir, it was probably 

 correct to apply the term "tuberculum" in the sense in which "tri- 

 gonum" is now used. Gegbnraur had described the olfactory bulb as 

 attached to the rest of the cerebral hemisphere by the "tractus" (now 

 more accurately defined by the word "pedunculus") "olfactorius" and 

 the point of insertion of the latter (to the hemisphere) he called the 

 "tuberculum olfactorium". "When attention was directed to the lowlier 

 mammalian brains this term became transferred by most writers to the 

 prominent cap of cortex on the head of the corpus striatum, and during 

 the last fifteen years most writers have consistently employed it in 

 this sense, His's term "trigonum" being now applied to the structure, 

 which Gegbnbaur called "tuberculum". 



