CHAP. L] DIGESTIOX. 283 



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. 

 Recent observations however seem to shew that when the morsel 

 is not large and especially when the substance swallowed is liquid, 

 the movement of the back part of the tongue is 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 the con- 

 strictors have become paralysed by section of their motor nerves. 



Deglutition therefore, though a continuous 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 move- 

 ment is as rapid as possible. The third stage is the descent through 

 the grasp of the constrictors. Here the food has passed the respira- 

 tory orifice, and in consequence its passage may again become com- 

 paratively slow, or, as we have seen, may continue to be rapid. 



The first stage in this complicated process is undoubtedly a 

 voluntary action. The raising of the soft palate and the approxi- 

 mation of the posterior pillars may also be, at times, voluntary, 

 since they have been seen, in a case where the pharynx was laid 

 bare by an operation, to take place before the food had touched 

 these parts; but the movement may take place without any 

 exercise of the will or presence of consciousness. And indeed the 

 second stage taken as a whole, though some of the earlier com- 

 ponent movements are, as it were, on the borderland between the 

 voluntary and involuntary kingdoms, must be regarded as a reflex 

 act. The third and last stage, whatever be the exact form which it 

 takes, is undoubtedly reflex ; the will has no power whatever over 

 it and can neither originate, stop, nor modify it. 



Deglutition in fact as a whole is a reflex act ; it cannot take 

 place unless some stimulus be applied to the mucous membrane of 

 the fauces. When we voluntarily bring about swallowing move- 

 ments with the mouth empty, we supply the necessary stimulus 

 by forcing with the tongue a small quantity of saliva into the 

 fauces, or by touching the fauces with the tongue itself. 



In the reflex act of deglutition the afferent impulses originated 

 in the fauces are carried up chiefly by the glosso-pharyngeal, but 



