444 THE DIGESTIVE APPARATUS IN MAMMALIA. 



It therefore also belongs to the pharynx, and in contracting diminishes that 

 cavity. 



Pterygo-pharyngeus^ or Superior Constrictor (the palato-iihanjngeus 

 of Percivall). — This muscle is thin, wide, flat, and triangular. It arises from 

 the pterygoid process, whence its fibres diverge, some posteriorly, others in- 

 wardly. The former mix with those of the palato-pharyngeus, and comport 

 themselves like that muscle ; and the latter are united, on the median line, with 

 the analogous fibres of the opposite muscle, forming a kind of zone around the 

 origin of the Eustachian tube. This muscle is covered, externally, by a layer of 

 yellow elastic tissue, which is attached with it to the pterygoid bone ; afterwards 

 it is fixed to the superior border of the great cornu of the os hyoides, and is 

 even prolonged on the external surface of the muscle it covers to the thyroid 

 cartilage. 



The elasticity of this fibrous layer plays a certain part in the movements of 

 the hyo-laryngeal apparatus, in acting as a passive antagonist of its depressors. 



This muscle is, and can only be, a perfect constrictor o^ the pharynx, as it 

 diminishes the diameter of that cavity in every direction — the longitudinal 

 diameter, by means of its posterior fibres, which draw the thyroid cartilage 

 forward ; and its transverse diameter, by the band thrown around the orifice of 

 the Eustachian tubes (Figs. 220, 6 ; 251, 1 ; 252, 8). 



Hyo-pharyngeus, or Middle Constrictor ; Thyro-pharyngeus, or 

 First Middle Constrictor ; and Crico-pharyngeus, or Inferior Con- 

 strictor. — The two last of these muscles only form one in Man — the inferior 

 constrictor of the pharynx. They are three muscular bands which terminate 

 above the pharynx, on a median fibrous fold, sometimes wide enough to look 

 like a triangular aponeurosis. The first band arises from the cornu of the os 

 hyoides ; the second, from the external surface of the thyroid cartilage ; the 

 third, from the superficial face of the cricoid cartilage. 



These are universally regarded as constrictors (Figs, 220, 7, 8, 9 ; 251, 4, 

 5, 5). 



In attentively examining the crico-pharyngeus muscle of different authorities, 

 two fasciculi are recognized, which can be distinguished by the direction of their 

 fibres. The deep band passes to the origin of the oesophagus ; it will be described, 

 as well as the muscle hitherto np'iied the arytaeno-pharyngeus, when the oesophagus 

 is referred to. 



Stylo-pharyngeus. — A narrow band which descends from the inner surface 

 of tlie styloid bone on the side of the pharynx, where it is confounded with the 

 pterygo-pharyngeus. It passes beneatli the hyo-thyroid and crico-pharyngeal 

 muscles, and, spreading out like a fan, becomes attached to the upper border of 

 the thyroid cartilam^ and mucous membrane of the pharynx, above the posterior 

 pillar of the soft palate. 



It narrows the longitudinat diameter, and it has been also regarded as a 

 dilator ; though the disposition of the parietes o/ the pharynx and the feeble 

 volume of this muscle, scarcely allow it to play any efficacious part in the 

 dilatation of that cavity. At most, it can only produce a very slight infundi- 

 bulum where it is inserted. The real dilating agent of the pharyngeal cavity is 

 the alimentary bolus, which is pushed into it by the action of the tongue and 

 soft palate (Fig. 251, 2, 2). 



It is not rare to find a second stijlo-pharyngeus muscle, terminating at the 

 same point as the first, but proceeding from the inferior extremity of the large 



