x NERVES 461 



Each spinal nerve arises, as in the frog (p. 163), by 

 two roots, which, however, are not in the same trans- 

 verse plane, the dorsal root being slightly anterior to 

 the corresponding ventral root, so that successive dorsal 

 and ventral roots of either side alternate with one 

 another. As already mentioned (p. 440), the two roots 

 of each nerve pass out from the neural canal indepen- 

 dently, uniting on the outside of the canal to form the 

 spinal nerve-trunk. Plexuses in connection with the 

 nerves supplying the paired appendages are much 

 simpler than in the frog (pp. 161 and 162). A sympa- 

 thetic system (p. 162) is represented. 



The origin and distribution of the cerebral nerves is 

 in the main similar to that already described in the case 

 of the frog (p. 163), the chief differences, characteristic 

 respectively of pulmonate and of branchiate Vertebrates 

 in general, being as follows. 1 



In fishes, there are certain nerves, usually counted as 

 belonging to the facial and vagus, which supply the 

 sensory canals of the integument (Fig. 125, VII op, b, e.m, 

 and X /) : these organs (pp. 432 and 464) are not present 

 in terrestrial forms, and their nerves are consequently 

 also wanting. The vagus, moreover, gives off a series 

 of branchial branches (br. 1 - 4 ) to the gills instead of 

 a pulmonary branch, and the glossopharyngeal (IX) is 

 also a branchial nerve. 



The olfactory nerves (Fig. 125, I) arise from the olfactory 

 lobe, which is situated in a large aperture in the skull 

 communicating between the cranial and olfactory cavities. 

 The optic nerve (II) is continued outwards from the optic 

 chiasma, and passes through a foramen in about the middle 

 of the orbit, towards the ventral side (compare Fig. 112). 

 The oculomotor (III), arising from the crura cerebri, makes 

 its exit from the skull a short distance behind and slightly 



1 The nature of a small paired nerve in dogfishes and certain other 

 fishes which arises from the prosencephalon and passes to the 

 nasal mucous membrane is not understood. 



