THE HEAD AND NECK 357 



visceral arch), and down this there passes the facial nerve. In a 

 similar way, the glossopharyngeal nerve passes down the 3rd 

 visceral arch, behind the 1st gill-slit ; and a branch of the vagus 

 runs down each of the 4th, 5th, 6th, and 7th visceral arches. 



Since the dorsal nerve-roots are segmental in arrange- 

 ment, the visceral arches are segmental also, for they correspond. 

 This means that the spiracle and gill-slits are intersegmental 

 in arrangement. It must be remembered, however, that this 

 segmental arrangement of the visceral arches is not the same 

 thing as the primary and fundamental segmentation of the 

 somites, because the visceral arches lie in the lateral-plate 

 mesoderm (not in the segmented vertebral plate). The cavities 

 enclosed in the mesoderm of the visceral arches are really 

 part of the originally continuous splanchnocoel, and not 

 myoccelic cavities. This is important, for it explains why the 

 muscles to which the mesoderm of the visceral arches gives 

 rise are innervated by dorsal and not ventral nerve-roots, 

 although they are striped and voluntary. Ventral nerve-roots 

 only innervate somatic striped muscles derived from the 

 segmented myotomes of the vertebral plate. Muscles formed 

 from the visceral mesoderm (inner wall of the splanchocoelic 

 cavity) in the region behind the gill-slits are of course the smooth 

 muscles of the gut, innervated by the autonomic system. That 

 the muscles of the visceral arches should differ from these 

 latter in being striped and voluntary is due to the fact that, 

 unlike them, they are attached to skeletal structures. These 

 skeletal structures support the jaws and the branchial arches, 

 and their movements are involved in the processes of biting 

 and breathing, which are related to the outside world. Smooth 

 muscles are only related to the events which go on inside the 

 animal. 



Since the visceral arches correspond to the segmentation 

 of the body, the structures in them correspond also. These 

 consist of the skeletal elements just mentioned, and of the 

 blood-vessels which run up round the gut from the ventral 

 to the dorsal aorta. So Meckel's cartilage and the quadrate 

 correspond to the trigeminal nerve and the 2nd segment, and 

 the hyomandibula and ceratohyal correspond to the facial 



