THE MUSCULAR MECHANISMS OF DIGESTION. 295 



depressed over the larynx. The thyroid cartilage is now, by the action of 

 the laryngeal muscles, suddenly raised up behind the hyoid bone, and thus 

 assists the epiglottis to cover the glottis. 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 epi- 

 glottis. 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 

 downward, thrust it into the oesophagus, along which it is driven by a similar 

 series of successive contractions 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 downward 

 after the food has passed by them, as if to complete the act and to insure 

 that nothing has been left behind. Deglutition in this fashion still remains 

 possible after these constrictors have become paralyzed by section of their 

 motor nerves. 



When a second act of deglutition succeeds the first with sufficient rapid- 

 ity, 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 oesophagus, 

 being forced into the stomach by the sequent peristaltic contraction of the 

 oesophagus. 



It will be seen from what has been said that deglutition, though a con- 

 tinuous act, may be regarded as divided into three stages : The first stage 

 is the thrusting'of the food through the isthmus faucium ; this may be either 

 of long or short duration. The second stage is the passage through the 

 upper part of the pharynx. Here the food traverses a region common both 

 to the food and to respiration, and in consequence the movement is as rapid 

 as possible. The third stage is the descent through the grasp of the con- 

 strictors. Here the food has passed the respiratory orifice, and in conse- 

 quence its passage again becomes comparatively slow, except in case of 

 fluids and small morsels, when, as we have seen, it may continue to be rapid. 

 The passage along the oesophagus may, perhaps, be regarded as constituting 

 a fourth stage ; but it will be more convenient to consider the cesophageal 

 movements bv themselves. 



