THE SEGMENTATION OF THE HEAD. 



143 



third cavity lies above the hyoid arch, and is finally con- 

 verted into the external rectus muscle, controlled by the abdu- 

 cens nerve. The fourth cavity, also, lies partly in the hyoid 

 arch. The rest follow in regular sequence, interrupted only by 

 the auditory organ. Of these latter the fourth and fifth degen- 

 erate, the sixth produces a few small muscle fibers, while the 

 rest unite in forming the ventral prolongation of the sterno- 

 hyoid muscle. From these facts Van Wijhe concludes that 

 there are nine segments in the head, and that the hyoid arch 

 is really double. 



The nerves are carefully studied in connection with these 

 somites. The olfactory and optic nerves are omitted from the 

 discussion, since, among other points, they are in front of the 

 segments. It is interesting to note that Van Wijhe shows 

 that the optic nerve is really the most anterior of all the cranial 

 nerves. With the remaining nerves the attempt is made to 

 distinguish dorsal and ventral roots. The results can be seen 

 in this condensed copy of Van Wijhe's table : — 



Since Van Wijhe's time numerous attempts have been made 

 to add to his structure, and in most of these the nerves have 

 been made especial objects of study, and the results would be 



