574 Dynamic Theory. 



matter mixed with the white fibres of the cerebellum ; and there is usu- 

 ally a cavity, or ventricle, in the cerebellum of the osseous fishes which 

 connects with the fourth ventricle. The Eel, Tench and Lepidosteus 

 ( Gar pike ) are, however, destitute of this ventricle, their cerebellum 

 being solid. There is no arbor vitae in the cerebellum of any fish ex- 

 cept the Tunny and Shark. 



The part of the brain which includes the cerebellum and the medulla 

 oblongata, with their appendages, is denominated, by Owen and others, 

 the Epencephalon. The greater part of the nerves controlling the ac- 

 tive movements of the body, have their roots in this part of the brain. 

 The great trigeminum and the vagus, or pneumogastric, which are par- 

 ticularly of great importance to the motor activities, originate near each 

 other in the restiform tracts of the medulla oblongata. In the fishes, 

 especially the active ones, these nerves are very large, and in some cases 

 their roots are swollen into large masses called lobes. Trigeminal lobes 

 are shown in the Chimera Monstrosa, fig. 282. In the Skate these 

 lobes are very large and convoluted. In the Torpedo these lobes are 

 blended with the vagal lobes, and are of vast size. They are, in this 

 case, called electric lobes. These lobes are not much developed in most 

 of the osseous fishes. There is a band of fibres stretching across be- 

 neath the cerebellum, called the restiform commissure. This is very 

 large in the Carcharias ( Shark ), amounting almost to an additional lobe 

 of the cerebellum. There cannot be any doubt that the epencephalon 

 is the condensing and directing apparatus for the physical movements of 



FIG. 283. Brain of J^epidosteus Semiradiatus. 

 I. Top view. e. Optic Lobes. 



II. Underside. 



a. Olfactory Nerves. 



6. Optic Nerves. 



3. Optic Lobes. 

 f. Medulla Oblongata, 

 g. Cerebellum. 

 A. Pituitary Gland. 



c. Cerebral Hemisphere, i. Hypoaria, 

 d. Optic Chiasm. Lobes. 



. 

 or Inferior 



the animal. The minor appendages depend 

 for the state of their development upon the 

 peculiar habits of activity affected by the 

 animal. 



The next division of the vertebrate brain 

 is often called the mesencephalon. This di- 

 vision in the fishes includes the optic lobes, 

 which are built on top, or on the dorsal side, 

 of the spinal medullary stem, and in most fishes two other bulbs, which 

 are developed on the opposite or under side, and which are called Hypo- 

 aria, or lobi inferiores. The hypoaria are well developed in the cod, 

 fig. 289. They are formed upon the anterior or under side of the an- 

 terior pyramids. In the cod, and some others, they each have a hollow 

 inside, or ventricle, called the L^poarian ventricle. Underneath the hy- 

 poaria is a median and single body, vascular and medullary, called the 



FIG. 283. 



