x NERVES 461 



with the third ventricle and anteriorly 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. As already men- 

 tioned (p. 440); the two roots of each nerve pass out from 

 the neural canal independently, uniting on. the outside of 

 the canal to form the spinal nerve. Plexuses in connection 

 with the nerves supplying the paired appendages are much 

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

 system 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. 



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, VIIop, b, e.m, and X!) : 

 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. 1 4 ) to 

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

 pharyngeal (IX} is also a branchial nerve. 



The olfactory nei-ves (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 octdomotor (III), arising from the crura 

 cerebri, makes its exit from the skull a short distance behind and 

 slightly above the optic nerve. The pathetic (IV), coming off from the 



