41 8 THE ANIMAL KINGDOM. 



branching primitive fibrils, and are remarkable for the extreme thickness 

 of their sheaths. The neural aspect of the plate is negative electrically. 



The brain fills the brain-case in embryonic Fish, but it finally 

 occupies a very small part of that cavity, owing to the disproportionate 

 growth of the cranial walls. The space left is filled by a fibrous and 

 fatty arachnoid tissue. The cerebro-spinal membranes develope pigment 

 cells, as in Amphibia. The nerve-centres are smaller in relation to the 

 bulk of the body in this than in any other Vertebrate class, the ratio of 

 brain to body being as low as i to 3000, and the spinal cord of a Sturgeon 

 weighing 120 Ibs. being no thicker than that of a Frog. The brain con- 

 sists of the usual parts. The olfactory lobes are connected to the hemi- 

 spheres by tracts of greater or less length in Elasmobranchii (with rare 

 exceptions) and a few Teleostei, e. g. Cyprinoidei. They are hollow in 

 Ganoidei. The cerebral hemispheres are the largest part of the brain in 

 Dipnoi, and as a rule in Elasmobranchii ; the corpora bigemina (optic lobes, 

 mesencephalon) the largest in Teleostei. The hemispheres are solid in the 

 last-named order, nearly or quite solid in Elasmobranchii, but contain 

 large ventricles in Ganoidei and Dipnoi. In Ceratodus they have the 

 greatest relative breadth, in Protopterus the greatest depth, in the class ; 

 and in the last-named the two lobes are separated nearly to their base. 

 On the contrary, the division into two lobes is scarcely indicated in Elasmo- 

 branchii. The region of the thalami optici is always small, and is hidden 

 dorsally by the hemispheres and optic lobes in Teleostei. The infundibulum 

 is large, and bears lobi inferiores (hypoaria) except in Sturgeons, Poly- 

 pterus and Dipnoi ; they are solid in Rays, hollow in other Elasmobranchii 

 and Teleostei. It bears also a saccus vasculosus in these two orders. The 

 pineal gland is long, filamentous, and tubular in Elasmobranchii, Acipenser, 

 and Ceratodus, and is attached anteriorly to the cranial roof. Its size 

 varies much in other Fish. The pituitary body is excessively large in 

 Polypterus and Protopterus, and is sometimes pedunculate, e.g. in Lophius 

 (the Angler), among Teleostei. The mesencephalon is undivided in Proto- 

 pterus ; in other Fish it is bilobed, and the lobes contain large ventricles 

 into which the cerebellum projects in Teleostei. The cerebellum is small, 

 and like that of the Frog in the Sturgeon, Polypterus and Dipnoi. It is of 

 fair size in Teleostei ; often large, lobed, and even convoluted in Elasmo- 

 branchii. It is foliate in the Tunny (Teleostei). The medulla is very long 

 and the fourth ventricle of great extent in Elasmobranchii, Acipenser, Poly- 

 pterus and Dipnoi. The restiform tracts are often convoluted anteriorly, 

 forming trigeminal lobes in Elasmobranchii and some Teleostei, or enlarged 

 into vagal lobes, e.g. in Cyprinoidei. 



The two optic nerves cross one another in Teleostei, the right nerve 

 supplying the left eye, the left the right eye. In other Fish there is a 

 chiasma. The fourth (trochlear) and sixth (abducens) nerves appear to be 



