Brain and Pineal Structures of Polyodon folium. 305 



The hind-brain is the largest division of the brain in 

 Polyodon. It is large also in Acipenser ruhieundus, but 

 the disproportion is not so great in the latter. In the 

 shovel-fish it is, in front, more than twice the width of 

 the mid-brain. The extensive fourth ventricle is com- 

 pletely covered dorsally by a thick, tough, black epi- 

 thelial membrane, with strongly plicate inner surface. 

 When this membrane is removed the ventricle is seen to 

 be widely open and bounded largely by the conspicuous 

 restiform tract. The lobe of the cerebellum, already 

 mentioned as the valvida cei^ebelli^ can be seen extending 

 into the ventricle from the front, its forward extension, 

 already noted, occupying the posterior part of the ven- 

 tricle of the mid-brain. 



The trigeminal group of nerves, arising from the lateral 

 wall of the medulla, is made up of three well-defined parts 

 in Polyodon. The most anterior division is a slender 

 nerve* about as large as the oculo-motor, which extends 

 obliquely outward and forward, crossing over the oculo- 

 motor and passing out towards the snout. It seems to 

 be throughout entirely independent of the other divisions 

 of the fifth nerve. The second division is much larger 

 and, arising just behind the first, extends outward and 

 forward, parallel with the first, until it joins the third 

 division. The latter is the largest division of the three, 

 and arises from the restiform tract, some distance be- 

 hind the second division and well up on the side wall of 

 the medulla. At first it extends downward and forward, 

 then outward over the large second division. 



The seventh nerve (facial) looks in Polyodon like a 

 posteriorly directed branch of the trigeminal. It m-ises 

 from the side of the medulla beneath the third division 

 of the fifth, with which it is at first very closely bound 

 up, extends forward and outward, and then turns ab- 

 ruptly backward, leaving the fifth at once. 



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♦Kamus opthalmicus superflcialis, according to Collinge. 



