378 HEAD AND NECK 



more or less directly posteriorly, forms its anterior wall in its 

 lower part. Above this is the isthmus of the fauces, or 

 the opening into the mouth, limited on either side by the 

 glosso-palatine arch. These arches may be regarded, there- 

 fore, as the lateral boundary lines between the mouth and the 

 pharynx. On the lateral wall of the oral pharynx the pharyngo- 

 palatine arch forms a prominent fold which is gradually lost 

 as it is traced downwards. Within this fold is the pharyngo- 

 palatine muscle. This is an important relation, because the 

 posterior palatine arches form the lateral boundaries of 

 the pharyngeal isthmus, and by the contraction of the 

 pharyngo-palatine muscles the two posterior arches can be 

 approximated until the opening of the isthmus is obliterated ; 

 the passage of food and fluids from the oral pharynx into 

 the naso-pharynx is thus prevented. 



The arcus glosso-palatinus and the arcus pharyngo-palatinus 

 form, on each lateral wall of the oral pharynx, the anterior and 

 posterior limits of a triangular interval in which is lodged the 

 tonsil. The upper part of this area, above the level of the 

 tonsil, presents a small depression termed the supra-tonsillar 

 fossa. 



In the child, and not uncommonly in the adult, a triangular 

 fold of mucous membrane, the plica triangularis, extends 

 posteriorly from the lower part of the glosso-palatine arch 

 and the base of the tongue across the surface of the tonsil. 

 The upper border of the fold may be free or it may become 

 attached to a greater or- less extent to the surface of the 

 tonsil. 



Pars Laryngea. The laryngeal portion of the pharyngeal 

 cavity diminishes rapidly in width to the level at which it be- 

 comes continuous with the oesophagus. In its anterior wall, from 

 above downwards, may be seen: (i) the epiglottis; (2) the 

 superior aperture of the larynx with the recessus piriformis on 

 either side ; and (3) the posterior surfaces of the arytsenoid 

 and cricoid cartilages, covered with muscles and mucous 

 membrane. 



Aditus Laryngis. The superior aperture of the larynx, 

 situated below the pharyngeal part of the tongue, is a 

 large, obliquely placed opening which slopes rapidly from 

 above downwards and posteriorly. It is somewhat triangular 

 in outline, and the basal part of the opening, placed above and 

 anteriorly, is formed by the free border of the epiglottis. 



