210 PISCES. 



(Gymnotus) the mesocerebrum is very strikingly developed. A lar- 

 ger lobe, divided by a median groove, arises from the optic lobes by 

 sloping semilunar-shaped roots, and forms a conical eminence that 

 extends far forward. The whole brain from this derives a peculiar 

 appearance ; it is in this ganglion that the roots of the nerves des- 

 tined to supply the electric organ arise, so that it may be termed 

 lobus electricus. 



A median lobe is developed in a similar manner in Echeneis re- 

 mora, and probably contains the nervous elements for the supply of 

 those remarkable organs situated upon the skull, by means of which 

 this fish adheres to ships, stones, and other bodies. 



Behind the cerebellum, and to the side of the fourth ventricle, are 

 situated frequently a pair of posterior lobes, which are occasionally 

 further subdivided into a special pair of lobes for the roots of the 

 nervi vagi. They are very small arid rudimentary in many Fishes, 

 and are therefore easily overlooked ; but are most developed in the 

 Carps, being as large, or even larger than the optic lobes. Other 

 Cyprinidae likewise possess these lobes of the nervi vagi, and they 

 appear also to contain the special nervous elements for a very irri- 

 table and contractile organ situated at the base of the cranium above 

 the pharyngeal bones, and which will be described more minutely 

 in speaking of the organs of taste. The lobes of either side are 

 connected by transverse white fibres, forming a kind of commissure 

 upon the under surface of the medulla oblongata. There frequently 

 occurs, as in many also of the Carp tribe, a single median ganglion, 

 which rests upon the floor of the fourth ventricle. This ganglion is 

 of largest size in the common Carp, and appears to exist also in the 

 Sheat-fish (Silurus). 



In many Fishes, e. g., in Trigla, from three to five ganglia are 

 constantly interposed behind the already-mentioned lobi posteriores, 

 near to the medulla oblongata, resting upon its upper tracts or cor- 

 pora restiformia ; they are also in connexion with peculiar struc- 

 tures, namely, with the large digitiform rays that are given off from 

 the pectoral fins in the Triglae, and which receive nerves of pro- 

 portionate size. Besides this, great diversities occur in the relative 

 proportions of the several parts of the brain ; thus the cerebellum is 

 in some cases greatly developed, in others very small. It is largest 

 indeed in the Tunny-fish (Scomber thynnus), and perhaps generally 

 in the Scomberidse, at least in S. scomber, where it forms a lobe 

 covering the mesocerebrum in the direction forward and upward. 



