yo NERVOUS SYSTEM OF VERTEBRATES. 



The ophthalamicus profundus nerve arises from the neural crest 

 over the mesencephalon; its root has shifted back to join that of 

 the trigeminus. The trigeminus arises over the metencephalon 

 and in cyclostomes remains permanently attached to that segment. 

 In all other vertebrates it moves back one segment to the first 

 neuromere of the myelencephalon. The facialis has shifted 

 back not only one segment, but two, leaving the neuromere between 

 it and the trigeminus without any nerve root. The shifting of 

 the VII is probably due to the shifting of the auditory sac which 

 has in part pushed it back and in part moved back past it. By 

 this movement the IX and X nerves are also affected. There 

 is reason to believe that the IX has moved back one segment in 

 cyclostomes, and that the IX and X have moved back two neuro- 

 meres in fishes and three in reptiles, birds and mammals. 



A word should be said regarding the complex character of the 

 X nerve. There is reason for thinking that each one of the epi- 

 branchial ganglia of the vagus represents an independent branchial 

 nerve, and that these nerves one after another beginning at the 

 caudal end of the gills have joined the nerve next anterior until 

 all have united into a single root. In the myxinoids this process 

 has gone one step farther than in other vertebrates, and the vagus 

 itself is joined to the glossopharyngeus. The actual shifting for- 

 ward and fusion of these nerves does not now take place during 

 the ontogeny, so that it is to be regarded as a very primitive feature 

 of vertebrate development. 



It is impossible in this book to follow further the evolution of 

 the vertebrate head. The accompanying tables and figures are 

 intended to make more clear the brief statements that have been 

 made. Figure 38 is meant to show the relations of the various 

 structures which enter into the primitive head segments. The 

 two tables give further facts which should be taken into account 

 in connection with this diagram. Table A is abbreviated from 

 the tables of Hoffmann showing the shifting of organs which 

 takes place during the development of the selachian head. Table 

 B is meant to show the elements present in each segment of the 

 head and the more important reductions and changes of position 

 which have taken place. 



