THE PHAEYNX 



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the formation of important structures. Thus, the first inner furrow becomes the middle-ear 

 cavity and Eustachian tube, the "closing membrane" persisting as the tympanic membrane 

 separating the inner from the outer furrow, which becomes the external auditory meatus. The 

 second inner furrow becomes the lateral recess of the pharynx, and its entodermal lining gives 

 rise to the tonsil. From the third inner furrow are developed the thymus and the inferior para- 

 thyroid gland, while the fourth gives rise to the lateral lobes of the thyroid and the superior 

 parathyroid glands. The isthmus of the thyroid is derived by a median ventral evagination 

 of the entoderm arising between the tuberculum impar and the second visceral bar. Another 

 median ventral evagination occurs at the level of the fourth visceral arch to form the respiratory 

 apparatus. 



Applied Anatomy. The internal carotid artery is in close relation with the pharynx, so that 

 its pulsations can be felt through the mouth. It has been occasionally wounded by sharp- 

 pointed instruments introduced into the mouth and thrust through the wall of the pharynx. 



FIG. 951. The anterior surface of the pharynx. (Sappey.) 



In aneurism of this vessel in the neck the tumor necessarily bulges into the pharynx, as this is 

 the direction in which it meets with the least resistance, nothing lying between the vessel and the 

 mucous membrane except the thin Constrictor muscle, whereas on the outer side there is the 

 dense cervical fascia, the muscles descending from the styloid process, and the margin of the 

 Sternomastoid muscle. 



The mucous membrane of the pharynx is very vascular, and is often the seat of inflammation, 

 frequently of a septic character, and dangerous on account of its tendency to spread to the larynx. 

 On account of the tissue which surrounds the pharyngeal wall being loose and lax, the inflam- 

 mation is liable to spread through it far and wide, extending downward into the posterior medi- 

 astinum along the oesophagus. Abscess may form in the connective tissue behind the pharynx, 

 between it and the vertebral column, constituting what is known as retropharyngeal abscess. 

 This is most commonly due to caries of the cervical vertebrae, but may also be caused by sup- 

 puration of a lymph node which is situated in this position opposite the axis, and which receives 

 lymphatics from the nares, or by cjumma or by acute pharyngitis. In these cases the pus may 



