Pike. 



COMPARATIVE ANATOMY OF THE ENCEPHALON. 



Fig. 27. 

 Cod, 



159 



Brains of Fishes, after Leuret; A, olfactive lobes or ganglia; B, cerebral hemispheres; c, optic lobes; D, 

 cerebellum; o I, olfactory nerve ; o p, optic nerve; p a, patheticus; m o, motor oculi; a b, abducens ; t r t, 

 trifacial ; fa, facial ; a M, auditory ; v a g, vagus ; 1 1 : tubercles or ganglia of the trifacial ; t r, tubercles of 

 the vagus. 



enlargements, 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 themselves. 

 214. The Optic Lobes of Fishes have no analogy whatever with the Thai- 

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

 apparent only. They are rather to be compared with the Tubercula Quadri- 

 gemina, which are the real ganglia of the Optic nerve. Their analogy is not 

 so complete, however, to these bodies in the fully-formed Brain of Man, as it 

 is to certain parts which occupy their place at an earlier period. The Third 

 Ventricle^ which is quite distinct from the Corpora Q,uadrigemina, is hollowed 

 out, as it we%, from the floor of the Optic Lobes of Fishes; and the Anterior 

 Commissure bounds its front : hence these must be considered as analogous to 

 the parts surrounding the Third Ventricle, as well as to the Corpora Cluadri- 

 gemina. This is made evident by the fact, observed by Miiller, that, in the 

 Lamprey, there is a distinct Lobe of the third ventricle, replacing the Optic 

 Lobes of other fishes, and partly giving origin to the optic nerves ; and a sepa- 

 rate vesicle, analogous to the Corpora Quadrigemina. With this condition, 

 the early state of the* Brain in the embryo of the Bird and Mammiferous 



