114 



THE BRAIN OF FISHES 



are received, and sent forth. From the whole length of 

 the spinal cord a series of nerves is given off on each 

 side, and each of them is connected therewith by an ante- 

 rior (or motor) and a posterior (or sensory) 

 root, the latter swelling into a more or 

 less distinct ganglion just where its fibres 

 begin to mingle with those of the anterior 

 root. This mode of connection of the 

 spinal nerves with the spinal cord exists 

 throughout the class of Fishes and also 

 in all other Vertebrates. 



Anteriorly the cord is continuous with 

 a slightly more swollen prolongation the 

 before-mentioned Medulla Oblongata 

 (fig. 47, D). Many very important nerves, 

 to which reference will subsequently be 

 made, are attached to this part. 



Growing from the back of the anterior 

 extremity of the medulla is a semi-ovoid 

 or tongue-like projection, which has been 

 already referred to as the Cerebellum. 

 Though single in appearance, it is really 

 double and composed of two symmetrical 

 halves. No distinct connection of nerves 

 with this body can be detected by the naked 

 eye. 



The cerebellum exists in its simplest 

 form in the parasitic Cyclostomes, in the 

 H , spinal cord; x, Sturgeon, and also in Polypterus and Le- 



olfactory nerve, di- . _ . 



vidingandpenetrat- pidosiren, where it appears merely as a 

 ing the plate of the s i m p} e bridge or commissure, crossing the 



ethmoid bone (Sol- * 7 



iy.) anterior and upper part of the medulla. 



In most osseous Fishes it is larger, and 



projects backwards over the medulla in the form of a 



FIG. 47. Brain of 

 the Pike. A, Olfac- 

 tory ganglia ; B, ce- 

 rebral lobes ; c, optic 



