148 ORGANIZATION OF THE HEAD v. 7- 



7. Segmentation of the vertebrate head 



The rhythmicity or metamerism seen in the cartilages can be traced 

 throughout the structure of the head. Although in higher vertebrates 

 the head appears as a distinct structure, separated from the body by 

 a neck, yet there is every reason to think that it has arrived at that 

 state by gradual modification of the anterior members of an originally 

 complete metameric series. The jaws, the receptor-organs, and the 

 brain have become developed at the front end of the body, producing 

 what zoologists conveniently if pretentiously call cephalization. 



The fundamental segmentation of the head is not very easily appar- 

 ent to superficial observation; the working out of its details is an 

 excellent exercise in morphological understanding. Recognition of the 

 segmental value of the various structures also makes them the more 

 easily remembered. For instance, the nerves found in the head have 

 been named and numbered for centuries by anatomists in an arbitrary 

 series: 



I. Olfactorius 

 II. Opticus 



III. Oculomotorius 



IV. Trochlearis (patheticus) 

 V. Trigeminus 



VI. Abducens 

 VII. Facialis 

 VIII. Acousticus 

 IX. Glossopharyngeus 

 X. Vagus 

 XI. Accessorius 

 XII. Hypoglossus 



Morphological study has shown that these nerves are not isolated 

 structures, each developed independently, but that they represent a 

 regular series of segmental dorsal and ventral roots of the head 

 somites. The satisfaction and simplification given by this generaliza- 

 tion is one of the clearest advantages of morphological insight. More 

 important still, such understanding of the morphology of a structure 

 shows us how to look for the morphogenetic processes that produce it; 

 such knowledge of how organs are made is an essential step in mending 

 or remaking them. 



The idea of the essential similarity of structure of the head and 

 trunk was early developed by Goethe, who tried to show that the 

 mammalian skull is a series of modified vertebrae. Unfortunately this 



