i8o A HISTORY OF FISHES 



the entire fish, that of the Pike (Esox) lAr,? whilst in some 

 of the Sharks it is relatively still smaller. It is a remarkable 

 fact that the Mormyrids of tropical Africa have a brain which 

 is a good deal larger in proportion to the size of the body than 

 in any other fish, that of Mormjyrus, for example, being between 

 /^ and mV of the weight of the entire fish, or twenty-five times 

 greater than that of the Pike. 



There is little more to add concerning the spinal cord, which 

 is very uniform in structure throughout the Selachians and 

 Bony Fishes. It usually extends the whole length of the body, 

 but is much shorter in some of the Globe-fishes ( Tetrodontidae) 

 and their allies. In the huge Sun-fish (Mola) it is remarkably 

 reduced, being actually shorter than the brain: in a specimen 

 two and a half metres long and weighing about a ton and a half 

 the cord was only fifteen millimetres in length. 



The nerves may be divided into two categories, spinal and 

 cranial, the former having their origin in the spinal cord, the 

 latter in the brain. The spinal nerves are metamerically 

 arranged, that is to say, their number is the same as that of the 

 vertebrae, through or between which they pass out. The 

 cranial nerves consist of ten pairs, which may be briefly 

 described (Fig. 71). The first or olfactory nerve is a purely 

 sensory one, and controls the sense of smell, connecting the 

 nasal organ with the olfactory lobe. The second or optic 

 nerve (II) is likewise sensory, and supplies the eye. In the 

 Gyclostomes each optic nerve runs from the optic lobe direct 

 to the eye of the same side; in the Selachians the two nerves 

 are fused together to form an optic chiasma; and in the Bony 

 Fishes the two cross each other below the brain immediately 

 after leaving the optic lobes, the nerve from the left lobe going 

 to the right eye and vice versa. The third, fourth, and sixth 

 are motor nerves, and their function is to supply and stimulate 

 the muscles which move the ey^s. The third or oculomotor (III) 

 starts from the lower surface of the brain, the fourth or trochlear 

 from the groove between the optic lobes and the cerebellum, 

 but the sixth or abducens (VI), like the remainder of the cranial 

 nerves, has its origin in the medulla oblongata. The fifth or 

 trigeminal nerve (V), and the seventh or facial (VII), are 

 mixed nerves, being partly sensory and partly motor. Both 

 have branches which are widely distributed over the snout 

 and jaws. The eighth or auditory (VIII) is another sensory 

 nerve and supplies the organ of hearing. The ninth or glosso- 

 pharyngeal (IX) is mixed, and has a branch which forks over 



