3i6 MECHANISMS OF THE DIGESTIVE TRACT. 



epiglottidean folds, in which are contained the two small cartilages of 

 Santorini and Wrisberg. During normal respiration this cavity is 

 widely open, and the arytenoids lie against the posterior wall of the 

 pharynx. When deglutition takes place, the arytenoid cartilages 

 leave the posterior wall of the pharynx, are rotated and moved inwards 

 and forwards, with their internal faces in contact. In consequence of 

 this movement of the hinder boundaries of the laryngeal opening 

 forwards, this opening assumes the form of a triradiate fissure, the 

 vertical limb being short, while the transverse limb is rounded, owing 

 to the pulling inwards of the margins of the epiglottis. Round the 

 central point of the triradiate fissure three prominences are to be 

 observed, namely, the cushion of the epiglottis, and on each side of this 

 the cartilages of Wrisberg. Thus, according to Stuart, closure of the 

 larynx is effected chiefly by the pulling of the posterior boundaries of 

 its orifice forwards, and not, as often described, by the movement of its 

 anterior boundary, the epiglottis, backwards. 



The muscles which are actively involved in this closure are the 

 external thyro-arytenoid and arytenoid, and the ary-epiglottidean and 

 the lateral crico-arytenoid muscles. The external thyro-arytenoid 

 muscle, which runs from the thyroid cartilage in front to the outer 

 surface of the arytenoid cartilage behind, pulls the latter forcibly 

 forwards and rotates it inwards. It is therefore, with the arytenoideus 

 muscle, the chief factor in the closure of the larynx. The function of 

 the ary-epiglottidean muscles is to make tense the folds of mucous 

 membrane which bound the entrance on each side. The lateral crico- 

 arytenoids aid the thyro-arytenoids by pulling the bases of the arytenoid 

 cartilages forwards and rotating them inwards. 



By the action of these muscles, both the true and false vocal cords 

 are approximated. This closure of the glottis is not, however, essential 

 for the carrying out of deglutition. Longet showed that swallowing 

 was performed without passage of food into the windpipe, even if the 

 vocal cords were held apart by passing a pair of forceps up between 

 them from an opening in the trachea. 



The approximation of the posterior to the anterior boundary of the 

 laryngeal opening is only rendered possible by the elevation of the 

 whole larynx under the hyoid bone, which is chiefly effected by the 

 thyro-hyoid muscles. Hence the act of deglutition is rendered impossible 

 by fixation of the larynx. 



Thus the openings into the posterior nares and into the larynx being 

 both closed, and the entrance from the pharynx into the oesophagus 

 widened by the pulling forward of the arytenoid cartilages and the con- 

 traction of the palato- and stylo-pharyngeus muscles, the bolus is shot 

 rapidly past these openings into the region of the middle and inferior 

 constrictors of the pharynx. If it is large, it is seized by these 

 muscles and thrust down into the oesophagus by the successive con- 

 traction of their fibres from above downwards. With a smaller bolus, 

 however, or when liquids are swallowed, the movement of the back part 

 of the tongue is sufficient to propel the substance past the constrictors 

 and through the oesophagus to its lower end, or even through the cardiac 

 orifice into the stomach, as shown by Kronecker and Meltzer. 1 In such 

 cases, the rapid passage of the bolus is followed by a contraction of the 

 constrictors and by a peristaltic contraction of the oesophagus, which 



1 Arch.f. Physiol., Leipzig, 1880, S. 446 ; and 1883, Suppl., S. 328. 



II 



