578 Dynamic Theory. 



called the Prosencepkalon, and it consists simply of the two bulbs called 

 the hemispheres. A few of the medullary fibres, which continue into it 

 from the spinal cord, pass on along its base into the most forward di- 

 vision of the brain, called the Rhinencephalon, which is composed solely 

 of the olfactory lobes and the medullary stem, which may connect them 

 with the cerebral lobes. The cerebral lobes vary greatly in size in dif- 

 ferent fishes. Thus in the Cod, Carp, G-lobefish, Bream, Perch, Stickle- 

 back, Gurnard, and Lepidosteus, they are smaller than the optic lobes. 



FIG. 290. 



FIG. 290. Brain of Lepidosiren (Mudfish.) 



(7. Cerebellum. M Medulla Oblongata. 



o. Optic Lobes. h. Protuberance of anterior pyramids. 



w. Pineal Gland. p. Pituitary Gland. 



P. Cerebral Hemispheres. 2. Optic Nerves. 



5, Olfactory Lobes. lv. Cross-section of Cerebrum, showing the 



1, Olf Olfactory Nerves. lateral ventricles. ( Owen.) 



In the Shark (fig. 286 ) they are as large as the cerebellum and optic 

 lobes together. In the Gar fish, Herring and Lump fish the cerebrum 

 is smaller, relatively, than in the Cod. In the Myxine it is smallest of 

 all. In the Polypterus and Lepidosiren it is larger than all the rest of 

 the brain. 



The cerebral lobes of fishes are composed chiefly of gray vascular 

 nerve matter, through which the white fibres radiate. The surface is 

 pinkish in color, and nodulated. In the Cod and Amblyopsis there are 

 two or three convolutions, but as a rule the cerebral hemispheres are 

 smooth, from the Myxine, in which they are smallest, to the Sharks, in 

 which they are among the largest, and the Lepidosiren and Polypterus, 

 which are among the most highly organized fishes. In general, these 

 lobes are solid, but there are ventricles in those of the Lepidosiren, and 

 these communicate with ventricles in the olfactory lobes. In the Shark 

 there is an elementary ventricle in the shape of a fissure in the under 

 anterior part of the lobe, into which a fold of a vascular membrane, an- 

 swering to the choroid plexus, is inserted. This fissure also extends 

 into the olfactory lobe. As a rule, the cerebral lobes of fishes are dis- 

 tinct from each other, and are connected "by a commissure which is hom- 

 ologous with the " anterior commissure," which, in more highly devel- 

 oped animals, connects the two hemispheres. In a few families of 

 fishes these lobes are not separated at all. 



Observing that the ventricles of the cerebral lobes of Lepidosiren and 

 Sharks are excavated in the body of the lobes themselves, it is doubtful 

 if these ventricles are the homologues of the lateral ventricles which oc- 

 cur in higher brains. The ventricles in the cerebellum, the optic lobes 



