DEVELOPMENT OF THE PHARYNX AND NECK. 



29 



off vessels to the gills, in which the blood is arterialized. In 

 the human embryo the blood passes directly through the aortic 

 arches. 



All that part of the human neck lying in front of the vertebral 

 column and between the mouth above and the thorax and 

 clavicles below, with the bounding walls of the adult pharynx, 

 is formed from the foetal visceral arches. A knowledge of the 



nasal pit 



artery of 1st arch 

 artery of 2nd arch 

 gill cleft 

 ventral aorta 

 gill cleft 



artery of 4th arch 

 ventral aorta 

 bulbus arteriosus 

 ventricle 



auricle 



Fig. 21 B. — Showing the position of the Heart, Visceral and Aortic Arches in a fish. 

 (Diagrammatic — after Gegenbaur.) 



transformation of the foetal to the adult pharynx is of the 

 utmost practical importance: it explains the occurrence of fis- 

 tulae and cysts found in the neck ; it accounts for the peculiar 

 courses taken by nerves, such as the recurrent laryngeal and 

 phrenic ; it explains the peculiar distribution of nerves to the 

 pharynx ; and throws light on the nature and anomalies of the 

 thymus, thyroid and tonsil. 



The Branchial or Visceral Arches. — The visceral arches 

 bound and form the whole thickness of the wall of the 

 primitive pharynx. Four arches, each bounded behind by a 

 depression, are to be recognised superficially on each side of 

 the pharynx of the fourth week human embryo (Fig. 21^4), 

 but behind the 4th cleft is a fifth arch, perhaps also a sixth 

 which, however, never become raised or superficially differentiated 



