302 A MANUAL OF PHYSIOLOGY 



contraction of the mylo-hyoids the larynx is pulled upwards and 

 forwards by the contraction of the thyro-hyoid muscle, and the 

 elevation of the hyoid bone by the muscles which connect it to the 

 lower jaw. The base of the tongue is simultaneously drawn back- 

 wards by the stylo- and palato-glossus. The lower or laryngeal 

 portion of the epiglottis is thus caused to come into contact 

 with the upper orifice of the larynx, occluding it completely, but 

 the pharyngeal portion projects beyond the larynx, and takes no 

 share in its closure (Eykman). The glottis is closed by the ap- 

 proximation of the vocal cords and the arytenoid cartilages. 

 The epiglottis, however, is not absolutely indispensable for 

 closing the larynx, since swallowing proceeds in the ordinary way 

 when it is absent. The morsel of food, grasped by the middle and 

 lower constrictors as it leaves the back of the tongue, passes 

 rapidly and safely over the closed larynx, the process being accele- 

 rated by the pulling up of the lower portion of the pharynx over 

 the bolus by the action of the palato- and stylo-pharyngei. 



The second or oesophageal portion of the involuntary stage is 

 a more leisurely performance. The bolus is carried along by a 

 peculiar ' peristaltic ' contraction of the muscular wall of the 

 oesophagus, which travels down as a wave, constricting the tube 

 and pushing the food before it. In front of the constricting wave 

 moves a wave of inhibition, so that the part of the oesophagus 

 into which the bolus is about to pass is always relaxed, while the 

 part behind it is contracted. This exact co-ordination of inhibi- 

 tion and contraction is the essential thing in peristalsis. When 

 the food reaches the lower end of the gullet the tonic contraction 

 of that part of the tube is for a moment relaxed by reflex inhibi- 

 tion, and the morsel passes into the stomach. Beaumont saw, in 

 the case of St. Martin, that the cesophageal orifice of the stomach 

 contracted firmly after each morsel was swallowed, and so did 

 the gastric walls in the neighbourhood of the fistula when food 

 was introduced by this opening. In the dog the whole process 

 of swallowing from mouth to stomach has been shown to occupy 

 four to five seconds, but the time is by no means constant. In 

 man the peristaltic wave requires about five to six seconds to 

 travel from the level of the glottis to the cardiac orifice. The 

 rate of movement is greater in the upper than in the lower portion 

 of the oesophagus. 



Such is the mechanism of deglutition when the bolus is of such 

 consistence and size that it actually distends the oesophagus. 

 But it has been shown that liquid food is swallowed in a different 

 way. The food lying on the dorsum of the tongue, suddenly put 

 under pressure by the sharp contraction of the mylo-hyoid 

 muscles, is shot rapidly down to the lower part of the lax oeso- 

 phagus, or, occasionally, some of it even into the stomach. So 



