CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 481 



being also raised so that they come very near to the false vocal 

 cords. The thyroid cartilage is, by the action of the laryngeal 

 muscles, suddenly raised up behind the hyoid bone, and the epi- 

 glottis is depressed over the larynx, the cushion at the base of the 

 epiglottis covering the rima glottidis. This movement of- the 

 thyroid can easily be felt on the outside. Thus, both the entrance 

 into the posterior nares and that into the larynx being closed, the 

 impulse given to the bolus by the tongue can have no other effect 

 than to propel it beneath the sloping soft palate, over the incline 

 formed by the root of the tongue and the epiglottis. The palato- 

 glossi or constrictores isthmi faucium, which lie in the anterior 

 pillars of the fauces, by contracting, close the door behind the food 

 which has passed them. 



When the bolus of food is large, it is received by the middle 

 and lower constrictors of the pharynx, which, contracting in 

 sequence from above downwards, thrust it into the oesophagus, 

 along which it is driven by a similar series of successive con- 

 tractions which we shall speak of immediately as peristaltic 

 action. This comparatively slow descent of the food from the 

 pharynx into the stomach, may be readily seen if animals with 

 long necks such as horses and dogs be watched while swallowing. 

 When however the morsel is not large or when the substance 

 swallowed is liquid, the movement of the back part of the tongue 

 may be sufficient not merely to introduce the food into the grasp 

 of the constrictors of the pharynx, but even to propel it rapidly, to 

 shoot it, in fact, along the lax oesophagus before the muscles of that 

 organ have time to contract. In such a mode of swallowing the 

 middle and lower constrictors take little or no part in driving the 

 food onward, though they and the oesophagus appear to contract 

 from above downwards after the food has passed by them, as if to 

 complete the act and to ensure that nothing has been left behind. 

 Deglutition in this fashion still remains possible after these 

 constrictors have become paralysed by section of their motor 

 nerves. 



When a second act of deglutition succeeds a first with suffi- 

 cient rapidity, the nervous changes which start the pharyngeal 

 movements of the second act appear to inhibit the cesophageal 

 movements of the first act; and when swallowing is repeated 

 rapidly several times in succession, the oesophagus remains quiet 

 and lax during the whole time, until immediately after the last 

 swallow, when a peristaltic movement closes the series. 



When the stethoscope is applied over the oesophagus, at 

 different regions, a sound is heard during deglutition ; sometimes 

 two sounds are heard. The first and most constant is coincident 

 with the passage of the bolus, and is due to this and to the muscular 

 sound of the contracting muscles. The later and less constant 

 sound appears to be caused by a quantity of air-bubbles with 

 which the bolus was entangled, lodged at the cardiac end of the 



