CRANIAL NEBVES. 



153 



The lumbo-sacral plexus shows in general, and more 

 particularly in Mammals, much greater variations than does the 



FIG. 127.- CEREBRAL NERVES AND BRACHIAL PLEXUS OF Salamandra atra. 



l' a , ophthalmic branch of the trigeminal ; V b , its maxillary branch ; F c , its 

 mandibular branch ; t h, entrance of the ophthalmic branch into the nasal capsule 

 and V a , its extension forwards to the snout ; VII, facial nerve ; VIl a , its hyo- 

 rnandibular branch ; F7/' J , its palatine branch, which enters the nasal capsule 

 at * ; Co, commissure between the facial and glossopharyngeal (IX ) ; IX a , branch 

 of the glossopharyngeal to tongue ; IX b , its pharyngeal branch ; X, vagus ; XI, 

 spinal accessory ; XII 1, hypoglossal (first spinal nerve, which becomes connected 

 peripherally with the second spinal nerve, XII 2} ; 1 to 5, the first five spinal 

 nerves ; PI. brack., brachial plexus ; Sy, sympathetic cord, showing a connection 

 with the spinal nerves at Sy 1 ; Or, orbit ; M, maxilla. 



brachial plexus. The nerves arising from it are spoken of as 

 obturator, crural, and sciatic. The latter divides up in the 

 hind-limb into a tibial and a fibular nerve. 



2. CRANIAL NERVES. 



As already mentioned, the cranial nerves become so much 

 modified in the course of development that their primary rela- 

 tions, as a rule, can be no longer recognised. Nevertheless, it is 

 important to understand these primary relations thoroughly before 

 pursuing our inquiries further. It must therefore be borne in mind 

 that the head is primitively composed of a series of metarneres, and 

 that the brain and skull are correlated genetically. 1 



We must now ascertain, as far as is possible in the present 

 state of our knowledge, to which individual metameres the different 

 cranial nerves belong (see Fig. 41). The latest researches on this 

 subject have reference mainly to Elasmobranch embryos, though the 

 results obtained have been confirmed in other Fishes (Cyclostomi, 

 Teleostei), and to a certain extent in Mammals also. 



1 For the appearance of dorsal ganglia of the cranial nerves in the embryo, compare 

 the chapter on the sensory organs of the integument, p. 166. 



