184 LECTURE VIII. 



fold of membrane or ' choroid plexus ' penetrating the fissure, which 

 is continued forward into the crus of the olfactory lobe. The lateral 

 ventricle is more extensive in the Lepidosiren, and is continued 

 directly into the olfactory lobe. 



The ' rhinencephalon ' {figs. 47. 49, 50. 54, 55. r) consists of 

 two lobes of grey matter, which receive the prolongations of 

 chiefly white fibres from the prosencephalon and its crura, and 

 give off the nerves to the olfactory capsule, whence they are termed 

 ' olfactory lobes,' ' tubera,' or ' ganglia.' The rhinencephala are solid 

 bodies, always distinct, wide apart from each other when remote 

 from, and in mutual contact when near to, the rest of the brain, 

 but never united by a commissure. The rhinencephalic crura 

 {figs. 47. 51. 55. z) vary exceedingly in length. In the Lepidosiren 

 {fig. 54.) they are feebly -indicated by a continuous indentation 

 circumscribing the base of each rhinencephalon (r), and defining 

 it from the anterior end of each prosencephalon (p.) in Polypterus 

 and Lepidosteus {fig. 49.), the indentation is deeper, and the attach- 

 ment of the base of the now pyriform I'hinencephalon sinks to the 

 prolonged crus or basis of the prosencephalon. It is from this sub- 

 stratum that the rhinencephalic crura are prolonged in all osseous 

 fishes ; in some they are so short that the rhinencephala are partly over- 

 lapped by the prosencephala ( Trigla), or rise into view immediately 

 in front of them {Amblgojjsis, Anguilla, Cottus, Cyclopteriis) ; but in 

 many fishes the rhinencephala are developed far in advance of the 

 rest of the brain, and their crura are prolonged close to the olfactory 

 capsules : this has led some excellent observers to deny the existence of 

 olfactory lobes in such fishes; but the rhinencephala are truly present 

 in both the Tetrodon*, the Cod and Carp; they are merely removed 

 to juxta-position with the olfactory capsules, with a concomitant 

 prolongation of their crura. These crura, so prolonged, have been 

 called ' olfactory nerves ' by those who, failing to appreciate the true 

 homology of the remote 'rhinencephala,' have described them as 

 ganglionic swellings of the ends of the olfactory nerves, f These 

 ganglions, Avherever situated, consist of proper nervous matter over 

 and above the mere radiation or expansion of the fibres of the so- 

 called ' olfactory nerves.' The true olfactory nerve quits the rhinen- 

 cephalon as a plexiform chord, or as a group of distinct fibres. If 

 the thick olfactory nerve of the Gurnard be compared with the thick 

 rhinencephalic crus of the Skate, or if the long olfactory nerve of the 

 Eel be compared with the long rhinencephalic crus of the Cod, their 



* Dr. Desmoulins (i.xxviii. t. i. p. 169.), has erroneously denied the existence 

 of the 'lobes olfactifs' in the Diodon ; but in other fishes both he and Mr. Solly 

 (lxii. p. 78.), have taken a correct view of the rhinencephala or ' olfactory tubercles.' 



j- Camper, lxi. p. 95. ; Cuvicr, xxiii. p. 321. 



