ENCEPHALON OF FISHES. 



281 



357. Commencing with FISHES, we find a series of four distinct ganglionic 

 masses, arranged in a line which is nearly continuous, from behind forwards, 

 with that of the Spinal Cord; of these, the posterior is usually single, and 

 on the median plane, whilst the others are in pairs. The posterior, from its 

 position and connections, is evidently to be regarded in the light of a Cere- 

 bellum ; and it bears a much larger proportion to the rest, in this class, than 

 in any other. The pair in front of this are not the hemispheres of the Ce- 

 rebrum, as their large size in some instances (the Cod for instance) might 

 lead us to suppose; but they are immediately connected with the Optic nerve, 

 which, in fact, terminates in them, and are therefore to be considered (like the 

 chief part of the cephalic masses of Invertebrated animals) as Optic Ganglia. 

 In front of these are the Cerebral Hemispheres, which are small, generally 

 destitute of convolutions, and possess no ventricle in their interior, except 

 in the Sharks and Rays, in which they are much more highly developed than 

 in the Osseous Fishes. Anterior to these is another pair of ganglionic en- 

 largements, from which the Olfactory nerves arise ; and these are, therefore, 

 correctly designated as the Olfactive tubercles or ganglia. In some instances, 

 these ganglia are not immediately seated upon the prolonged spinal cord, but 

 are connected with it by long peduncles ; this is the case in the Sharks ; and 

 we are thus led to perceive the real nature of the portion of the trunk of the 

 Olfactory nerve in Man, which lies within the cranium, and of its bulbous 

 expansion on the Ethmoid bone. Besides these principal ganglionic enlarge- 

 ments, there are often smaller ones, with which other nerves are connected. 

 Thus, in the Shark, we find a pair of tubercles of considerable size, at the 

 origin of the Trifacial nerves ; and another pair, in most Fishes, at the roots 

 of the Vagi. In some instances, too, distinct Auditory ganglia present them- 

 selves ; as in the Carp. 



Pike. 



Cod. 



Fig. 143. 



Fox-shark. 



Brains of Fishes. A, olfactive lobes or ganglia; B, cerebral hemispheres; c, optic lobes; D, cerebel- 

 lum ; ol, olfactory nerve ; op, optic nerve ; pa, patheticus ; mo, motor oculi ; ab, abducens ; tri, trifacial ; 

 fa, facial; vag, vagus ; tt, tubercles or ganglia of the trifacial ; tc, tubercles of the vagus. 



358. The Optic Lobes of Fishes have no analogy whatever with the Tha- 

 lami optici of Mammalia ; the connection of which, with the Optic nerves, is 



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