DEVELOPMENT OF THE PHARYNX 



217 



FIG. 206. DIAGRAM OF THE PHARYNX 

 OF A MAMMALIAN EMBRYO. 



oesophagus; p. b., postbranchial body; 

 t., thyreoid; th., thymus; tr., trachea; 

 i> 2, 3, 4, the pharyngeal pouches. 



S.t 



ectoderm at the bottom of a single funnel-shaped depression known as 

 the cervical sinus (Fig. 205, c.s.). This also wholly disappears normally, 

 but it may remain as a cervical fistula low down on the neck, and its 

 deeper parts may give rise to branchial cysts. Thus all the ectodermal 

 branchial grooves except the first normally 

 disappear before birth. 



The pharyngeal pouches, or entodermal 

 portions of the gill clefts, as they occur in 

 a mammalian embryo are shown in Fig. 

 206. The pharynx opens to the exterior 

 at the mouth, m, and divides posteriorly 



into the trachea, tr, and CeSOphagUS, Oe. 



In the median dorsal line it gives rise to 

 the anterior lobe of the hypophysis, cut off 

 at a. /., and in the median ventral line to 

 the thyreoid gland, t. This gland is a median structure, entirely sepa- 

 rate from the pharyngeal pouches. It grows down through the hind 

 part of the tongue, acquiring a position in front of the trachea. Its 

 branching terminal part becomes separated from its outlet by the ob- 

 literation of its duct (called the thy- 

 reoglossal duct). A blind pit, the 

 foramen cacum, permanently re- 

 tained at the back of the tongue, 

 marks the former outlet of the duct 

 (Fig. 207,7. c.}. Thus the thyreoid 

 gland is a detached clump of ento- 

 dermal tubules in front of the 

 trachea. 



The entodermal portions of the 

 gill clefts are four paired lateral 

 outpocketings. The first (Fig. 206, 

 i) extends to the auditory groove in 

 the ectoderm, and becomes the audi- 

 tory tube (Eustachian tube). The 

 pharyngeal orifice of this tube in the 



FIG. 207. A MEDIAN SB TION THROUGH THE . . . , . _,. , , ^ 



PHARYNX OF AN ADULT. (After Corning.) adult IS SnOWTl in rig. 2C>7 (0. pn.)\ 



P leal archre^^iglottisTfJ'cf.'fo'rame'n csecmn the OUter end of the tube expands tO 



i. s-t., supratonsillar fossa; o. ph., pharyngeal r . , . j_ r , i 



orifice of the auditory tube; pal., soft palate; r. f Orm the tympanic Cavity Of the Car, 

 ph., pharyngeal recess; s.t., sellaturcica (which i i i i 



contains the hypophysis); t. 1., lingual tonsil; and Will be further Considered With 

 tons., palatine tonsil; t. ph., pharyngeal tonsil. 



the sense organs. 



The second pharyngeal pouch (Fig. 206, 2) loses its connection with the 

 ectoderm and becomes a relatively shallow depression on the side of the 

 pharynx. At a certain stage it is in close relation with the orifice of the 



