NERVOUS SYSTEM. 211 



In Silurus glanis it is very large ; very small, on the contrary, in 

 Gobius, but rarely provided, e. g. Echeneis remora, or Sucking-fish, 

 with transverse grooves, as in the higher Vertebrata. 



A want of symmetry between the two sides of the brain occurs in 

 different Osseous Fishes ; it is present in the least degree in Gadus, 

 where the cerebellum is mostly placed more toward the right side. 

 In Pleuronectes this asymmetrical condition is carried to a greater 

 degree, being extended to the cerebral ganglia, the optic lobes (the 

 left of which is usually placed above the other), and still more to the 

 olfactory lobes and olfactory turbercles, which are much larger upon 

 the left or uppermost side than upon the right. 



In the structure of the brain, as in many other points of its organi- 

 zation, the Sturgeon makes the transition from the Osseous to the 

 true Cartilaginous Fishes. The medulla oblongata is broader ; the 

 spinal canal opens into a very long and patulous rhomboidal sinus, 

 which is bounded upon either side by the ridges formed by the pos- 

 terior columns of the chord or corpora restiformia, that enter, as its 

 crura, the broad but small cerebellum. In front of the cerebellum 

 are placed the two moderately large optic lobes, which form the 

 mesocerebrum, together with the narrow mass that inferiorly and 

 laterally includes the freely exposed third ventricle. The olfactory 

 lobes are considerably larger in proportion than in the Osseous 

 Fishes, already presenting more the form of hemispheres, are further 

 subdivided and continued into small olfactory tubercles. The very 

 large pituitary body rests, without any stem orinfundibulum, upon the 

 base of the brain. Upon the sides of the medulla oblongata are a pair 

 of posterior lobes, which appear to N* chiefly in connexion with the 

 root of the fifth pair of nerves. 



The brain varies also in form in the Plagiostomi (Rays and 

 Sharks), but not nearly so much in proportion as that of the Osseous 

 Fishes ; and from its agreeing more, even when very different in 

 external appearance, with the brain of Amphibia, it is easier to de- 

 termine the signification of its component parts. The brain is chiefly 

 characterized by the more considerable development of its hemi- 

 spheres, from which arise either thick or slender nerves of smell ; 

 the lobes of the hemispheres are actually blended into a common 

 mass, divided only above in the longitudinal direction ; they are 

 either solid, as in most of the Rays when they have grown old, or 

 they are provided with a ventricle, which is continued then into the 

 olfactory nerves, as is seen chiefly in the Sharks, as Scymnus, Acan- 

 thias. This mass is prolonged by means of a narrow part of the 



