284 ANATOMY OF VERTEBRATES. 



phalic crura, figs. 171, 175, 178, 187, z, vary exceedingly in length. 

 In the Lepidosiren, fig. 186, they are feebly indicated by a con- 

 tinuous indentation circumscribing the base of the rhinenccphalon, 

 R, and defining it from the anterior end of each prosencephalic 

 lobe, P ; in Polypterus and Lepidosteus the indentation is deeper, 

 and the attachment of the base of the now pyriform rhinence- 

 phalon, fig. 174, R, sinks to the prolonged cms or basis of the pros- 

 encephalon. From this substratum the rhinencephalic crura are 

 prolonged in all Osseous Fishes : in some they are so short that the 

 rhinencephala are partly overlapped by the prosencephala ( Trie/la), 

 or rise into view immediately in front of them (Amblyopsis, 

 Anc/uilla, fig. 176, R, Coitus, Cyclopterus) ; 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 cap- 

 sules. This has led to a denial of the existence of olfactory lobes 

 in such fishes ; but the rhinencephala are truly present in both 

 the Cod and Carp, fig. 178, R ; they are merely removed to juxta- 

 position with the olfactory capsules, with a concomitant pro- 

 longation of their crura. These crura, so prolonged, ib. z, 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. 1 

 These ganglions, wherever 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 rhinencephalon 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 rhin- 

 encephalic crus of the Chub, fig. 177, k, their respective difference 

 of structure will be readily appreciated. The crus is a compact tract 

 of medullary with a small proportion of grey matter ; the nerve 

 is a bundle of nerve filaments : the medullary tract of the crus 

 is fibrous, but the fibres are as fine as in the crus cerebri, and 

 much more numerous and less easily separable than in the true 

 olfactory nerve. In this there is no grey tract : it consists wholly 

 of comparatively large and readily separable white fibres, which 

 radiate at once upon the olfactory capsule ; the divergence and 

 radiation of the true end of the olfactory nerve is well seen in 

 the Lepidosiren, fig. 186, i, ol. In Sharks a ventricle is continued 

 to each rhinencephalon along its crus from the prosencephalon. The 



1 Camper, LXI. p. 95; Cuvier, xxur. p. 321. 



