DEVELOPMENT OF THE PHARYNX 



217 



FIG. 206. DIAGRAM OF THE PHARYNX 

 OF A MAMMALIAN EMBRYO. 



2, 3, 4i 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 oesophagus, oe. 

 In the median dorsal line it gives rise to 

 the anterior lobe of the hypophysis, cut off 



7 j ,v j i T 



at a. I., 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 ccecum, permanently re- 

 tained at the back of the tongue, 

 marks the former outlet of the duct 

 (Fig. 207, /. 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 



, ,. . , T / .. i \ 



adult IS ShOWn in rig. 2C>7 (0. P/l.)] 



the outer end of the tube expands to 



. . , . ., r . , 



lOrm the tympanic Cavity OI tile ear, 



... , . , . - . . 



and Will be further Considered With 



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 



FIG. ? 207. A MEDIAN SE TION THROUGH THE 

 PHARYNX OF AN ADULT. (After Corning.) 



f. s-t., supratonsillar fossa; o. ph., pharyngeal 

 orifice of the auditory tube; pal., soft palate; r. 

 ph., pharyngeal recess; s. t., sella turcica (which 

 contains the hypophysis); t. 1., lingual tonsil; 

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



