NERVOUS SYSTEM OF FISHES. 177 



Loach and Herring {fig- 52. i), and also in the Stui-geon and Chi- 

 masra, where they are closely connected with a thick vascular mass of 

 pia mater and arachnoid. The trigeminal lobes are large in the 

 Skate; enormous and blended with the vagal lobes in the Torpedo : 

 but in most Osseous Fishes (Lepidosteus, Cod,) they ax'C not developed 

 so as to merit the name of lobes. In the Cod the inner surfaces of 

 the restiform bodies project into the fourth ventricle, and obliterate 

 the fore part of the ' calamus ' by meeting above it ; this commissure, 

 which is beneath the cerebellum, I call the ' commissura restiformis ; ' 

 it is remarkably developed in the Carcharias, where it seems to 

 form a small supplemental cerebellum beneath the lai-ge normal one, 

 (_/%. 55. I)* In figure 48. the medulla oblongata is cut across, 

 the fourth ventricle exposed from behind, and the restiform com- 

 missure, /, is raised : it has an anterior and posterior median notch. 



The primary division of the brain, which consists of the medulla 

 oblongata with the cerebellum and other less constant appendages in 

 Fishes, is called the ' epencephalon : ' it is relatively larger, occupies 

 a greater proportion of the cranium, and is more complex and diver- 

 sified in this than in any of the higher classes of Vertebrata. 



The next succeeding primaiy division of the brain, is called the 

 ' mesencephalon : ' it is usually the largest division in Osseous Fishes, 

 and consists of two uj)per spheroidal bodies, called 'optic lobes 'f (o), 

 of two lower subspherical bodies, called 'hypoaria'if (w), with inter- 

 vening connecting walls enclosing a cavity, called the ' third ventricle,' 

 which is prolonged downwards into the pedicle of the ' hypophysis ' or 

 pituitary gland ( ^j), and upwards into that of the ' conarium ' or pineal 

 gland (/f ). The prepyramidal columns are continued forwards, along 

 the floor of the fourth ventricle, where they are covered by a thin 

 layer of medullary fibres, to the hypoaria and prosencephalon ; some 

 fibres blending with the wall of the third ventricle and the base of the 

 optic lobes. The transverse ' ansulate ' commissure, which unites 

 or crosses the prepyramids before they penetrate the hypoaria, is 

 very obvious in the Sturgeon and Perch, where it is figured by 

 Gottsche (la'II. pi. iv.. Jig. 7. I) : it may be regarded as the most 

 anterior of the arciform filaments, which feebly represent the pons 

 Varolii in fishes. The restiform columns are expended chiefly in 

 forming the walls of the third ventricle and the base and exterior 



* The medullary lamina which Valentin describes as crossing the posterior point 

 of the calamus in the Chlmajra, may be the homologue of the restiform commissure. 

 Miiller's Archiv. 28. tab. 2. fig. 8, 9. 



•j- ' Lobes creux,' Cuvier, xxiii. i. p. 310. But the cerebellum and hypoaria are like- 

 wise ' hollow lobes,' and the prosencephala are hollow in the I.cpidosiren and Sharks. 



\ ' Lobes inferieurs,' ib. 



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