II NERVES 39 



In the Urodela the first spinal nerve either remains separate, 

 or it joins the second spinal, forming with it and with a branch 

 from the third nerve the cervical plexus, which supplies the 

 muscles of the cervical region. The third, fourth, and fifth 

 nerves, and sometimes also the sixth, form the brachial plexus. 



In the Aglossal Anura N. spinalis I. mostly sends a fine 

 thread to the second spinal nerve, the rest supplies chiefly the 

 M. levator scapulae, in Pljyi the abdominal muscles also. In all 

 the other Anura this N. s})inalis I. is lost ; occasional vestiges 

 have been reported in Bnfo vulgaris and Bana cateshiana, and 

 remnants of it may possibly be found in Pelobatidae and Discoglos- 

 sidae. The first actually persisting nerve of the Phaneroglossa is 

 consequently N. spinalis II. 



The brachial plexus is composed as follows : — FijJa, N. 

 spinalis II. and III. ; Xmopus, and Phaneroglossa, N. spinalis 

 III. and IV., with a small liranch from tlie second ; the next 

 following three nerves, numbers Y., YL, and YII., behave like 

 ordinary trunk nerves. 



The pelvic plexus of the Phaneroglossa is formed in Bana by 

 the YII I. + IX. + X. + Xlth nerves, the tenth issuing between 

 the sacral vertebra and the coccyx. In Bufo and Hyla the 

 plexus is composed of five nerves, the seventh spinal sending 

 a branch to it. Occasionally the twelfth nerve contributes a 

 small branch to the posterior portion of the plexus. This and 

 the eleventh nerve leave the coccyx by separate holes, thereby 

 indicating its composition. The rest of the spinal cord gives oft' 

 no more recognisable nerves, owing to its reduction during the 

 later stages of metamorphosis ; its terminal filament passes out 

 of the posterior end of the coccygeal canal. 



Concerning the cranial nerves it is necessary to draw atten- 

 tion to one point only. The last nerve which leaves the 

 cranium of the Amphibia is the vagus or tenth cranial nerve. 

 There is consequently no eleventh, and no twelfth or hypoglossal, 

 pair of cranial nerves. Their homologues would be the first and 

 second spinal nerves, but the whole tongue of the Amphibia, 

 with its muscles, is supplied by tlje glossopharyngeal, or ninth 

 cranial pair, and is morphologically not homologous with the 

 tonoue of the Amniota. 



