SEGMENTATION OF THE HEAD. 



439 



of the body-cavity develope into muscles and resemble the 

 muscle-plates of the trunk, and with these they must be identi- 

 fied, as has been already stated. As equivalent to the muscle- 

 plates, they clearly are capable of serving as very valuable guides 

 for determining the segmentation of the head. There are then 

 a pair of these in front of the mandibular arch, a pair in the 

 mandibular arch, and a pair in each succeeding arch. In all 

 there are eight pairs of these cavities representing eight seg- 

 ments, the first of them praeoral. As was mentioned above, 

 each of the sections of the head-cavity (except perhaps the first) 

 stands in a definite relation to the nerve and artery of the arch 

 in which it is situated. 



The comparative results of these three independent methods 

 of determining the segmentation of the head are in the sub- 

 joined table represented in a form in which they can be com- 

 pared : 



Table of the Cephalic Segments as determined by the Nerves, Visceral 

 Arches, and Head- Cavities. 



In the above table the first column denotes the segments of 

 the head as indicated by a comparison of the three sets of 

 organs employed. The second column denotes the segments as 



