180 COMPARATIVE ANATOMY 



crural, sciatic, and pudendic. The sciatic divides up in the hind- 

 limb into a tibicd and a fibular nerve. ^ 



2. CEREBRAL NERVES. 



The following twelve pairs of cerebral nerves can be distin- 

 guished, and of these the eleventh pair are plainly differentiated 

 only in the Amniota, and the twelfth are represented by the first- 

 spinal nerves in certain Fishes and in all Amphibians : — 



I. Olfactory. 



II. Optic. 



III. Oculomotor. 



IV. Pathetic or trochlear. 

 V. Trigeminal. 



VI. Abducent. 



VII. Facial. 



VIII. Auditory. 



IX. Glossopharyngeal. 



X. Vagus or pneumogastric. 



XI. Spinal accessory. 



XII. Hypoglossal. 



In their mode of early development the cerebral nerves resemble 

 the spinal nerves in many respects (p. 177), and a gradual tran- 

 sition between the two groups is indicated in the lower Vetebrata. 

 Certain of them, like the motor spinal nerves, arise as direct 

 ventral outgrowths from the embryonic brain (III, VI, XII, and^ 

 probably IV 2). Others, again (V and VII in part, VIII, IX, 

 and X), arise dorsally, primarily in connection with their indi- 

 vidual ganglia and becoming actually connected with the brains 

 secondarily : these must therefore, so far as they consist of sensory, 

 centripetal elements, be looked upon as homodynamous with the 

 dorsal roots of the spinal nerves. But it must be borne in mind' 

 that all these nerves, with the exception of the olfactory, optic, 

 and auditory, are of a mixed character, containing motor as well 

 as sensory fibres ; and a' further difference between them and the 

 dorsal roots of the spinal nerves (comp. note on p. 179) is seen in 

 the shifting of their origin to the ventral side of the brain during 

 development. 



A study of development shows that portions of the epiblast lying 

 peripherally to the brain take part in the formation of the ganglia of the 

 trigeminal, facial, auditory, and vagus nerves, and that each definitive 



^ In animals in which the extremities have disappeared, all traces of the 

 corresponding plexuses have also usually vanished : Snakes, however, still retain 

 remnants of them. 



^ The fourth nerve is peculiar in appearing from the dorsal surface of the 

 brain, but this is probably a secondary condition p. 184). 



