CHAPTER XXVIII 



THE HEAD AND NECK 



The structure of the head in an adult vertebrate animal is 

 somewhat complicated, and bears little resemblance to the 

 simpler segmented nature of the trunk-region. The segmenta- 

 tion is obscured, added to which there is the complication 

 introduced by the presence of the special paired sense-organs 

 (nose, eye, and ear) and of the gill-slits. The somites do not 

 all form straightforward myotomes as in the trunk, but give rise 

 to the eye-muscles ; and lastly, it is difficult to recognise the 

 segmental nerves because the dorsal and ventral nerve-roots 

 remain separate. Nevertheless, the head is built strictly on a 

 segmental plan, and it is easy to unravel its structure by 

 considering early stages of development. 



The embryo of the dogfish, for example, passes through a 

 stage in which the mesoderm on each side of the body is 

 segmented into a complete row of somites, from the front to 

 the hind end of the body. There is no difference between 

 the somites of the future head-region and those of the trunk, 

 and they grade insensibly into one another. The ist somite 

 is, however, peculiar in that it is connected with its fellow of the 

 opposite side by a strand of mesoderm-cells which passes in 

 front of the tip of the notochord. Such a connexion would 

 be impossible between somites situated farther posteriorly, 

 because the notochord separates those of one side from those 

 of the other. The ist somite is called the premandibular 

 somite, and it is innervated by a ventral nerve-root : the 

 oculomotor. The 2nd somite is rather larger than the others, 

 it is called the mandibular somite and is innervated by the 

 trochlear nerve. The 3rd somite is the hyoid somite, and it is 



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