IX NERVES 445 



communicate posteriorly with the third ventricle and ante- 

 riorly are continued into the olfactory lobes. 



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

 roots, which, however, are not in the same transverse 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 : they also alternate 

 on the right and left sides. As already mentioned (p. 424), 

 the two roots of each nerve passes out from the neural 

 canal independently, uniting on the outside of the 

 canal to form the spinal nerve. A sympathetic is repre- 

 sented. 



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 air-breathing and of branchiate Vertebrates in general, 

 being as* follows. 



In fishes, there are certain nerves, usually considered as 

 belonging to the facial and vagus, which supply the sensory 

 canals of the integument (Fig. 116, VIIop, b, e.m, and XI) : 

 these organs are not present in terrestrial forms, and their 

 nerves are consequently also wanting. The vagus, more- 

 over, gives off a series of branchial branches {br.'^ — *) to 

 the gills instead of a pulmonary branch, and the glosso- 

 pharyngeal {IX) is also a branchial nerve. 



The olfactory nei~ves (Fig. 116, I) arise from the olfactory lobe 01 

 each side, which is situated in a large aperture in the skull communicat- 

 ing between the cranial and olfactory cavities. The optic nerve (II) is 

 continuetl outwards from the optic chiasma, and passes through a 

 foramen in about the middle of the orbit, towards the ventral side. The 

 oculomotor (III), arising from the crura cerebri, makes its exit from the 

 skull a short distance bel^ind and slightly above the optic nerve. The 

 tathetic (IV), coming off from the dorsal side of the front end of 

 the bulb and supplying the superior oblique muscle, pierces the 



