THE PHARYNX. 



551 



the fourth week, the hinder visceral arches, and the pouches 

 separating them from one another, become much more clearly 

 defined : the pharynx also changes its shape, becoming much 

 wider in front, and narrowing posteriorly towards the oesophagus 

 (Fig. 238). 



The rnandibular, hyoid, and first and second branchial arches 

 are well defined (Fig. 238, MN, HY, BR 1 , and BR 2 ), the hyoid 

 arch being especially large. Both the internal visceral pouches, 

 and the external visceral grooves between the successive arches 

 are well marked. There is some doubt as to whether any of the 

 gill-clefts are actually open in the human embryo ; such evidence 



Fia. 238. The floor of the pharynx of a Human Embryo, twenty-three days 

 old, seen from above. Cf. Fig. 243, which represents the same embryo. 

 (From His.) x 30. 



A2, second aortic arch, in the hyoidean arch. A3, third aortic arch, in the first 

 branchial arch. A4, fourth aortic arch, in the second branchial arch. A5, fifth aortic 

 arch, in the third branchial arch. BB-1, first branchial arch. BR2, second branchial 

 arch. BB.3, third branchial arch. EB, membrane closing the hyomandibular cleft, 

 which Afterwards becomes the tympanic membrane. FL, furcula. HY, hyoid arch. 

 LG, lung. MN, mandibular arch. TTJ, tuberculum impar. 



as has been obtained points to the conclusion that none of the 

 clefts are really completed either at this or any other stage in 

 development ; the visceral pouches and the corresponding vis- 

 ceral grooves being always separated by thin partitions, as at 

 EB in Fig. 238. 



The second branchial arch, BR 2 , is bounded posteriorly by 

 the conspicuous and deep third branchial pouch ; immediately 

 behind this is a ridge, BR 3 , projecting into the cavity of the 

 pharynx, and bounding laterally the entrance to the ossophagus. 

 Although there is no external ridge on the surface of the embryo 

 corresponding to this internal ridge, yet its relations to other 



