MOVEMENTS OF (ESOPHAGUS. 259 



conveyed to the Medulla Oblongata^ or that portion of the spinal cord 

 which lies within the cranium, by afferent nerves that terminate in 

 it ; and, in immediate respondence to that impression, a motor impulse 

 is transmitted from it, which calls the muscles into the combined action 

 necessary to produce the movement. Now this contact also produces 

 a sensation, provided the brain be sound and awake, because nervous 

 fibres proceed from the mucous surface to the brain as well as to the 

 spinal cord ; but this sensation is not a necessary link in the chain of 

 actions, by which the movement is produced ; for the act of Deglutition 

 takes place during profound sleep, when all sensation is suspended, 

 and it may be excited even after the brain has been removed. It 

 seems to be voluntary, under ordinary circumstances, simply because it 

 is by an act of the will, that the matter to be swallowed is carried 

 backwards into contact with the fauces ; but that it is not so in reality, 

 is shown by the fact, that when this impression has once been made 

 with sufficient force, we cannot by any effort of the will, prevent the 

 action. We have a good example of this in the following circumstance, 

 of no very unfrequent occurrence. The tickling of the upper part of the 

 fauces with a feather is often practised to induce vomiting ; but if the 

 end of the feather be carried too low down, it excites the act of degluti- 

 tion instead ; the feather is grasped by the pharynx and drawn down- 

 wards ; and if it be not held tenaciously between the fingers, it is drawn 

 from them and carried downwards into the stomach. 



454. The carrying-back of the alimentary matter, so that it reaches 

 the fauces or upper part of the pharynx, is principally accomplished by 

 the tongue ; when it has passed the anterior palatine arches, these con- 

 tract and close over the tongue, so as to prevent the return of the food 

 into the mouth ; and at the same time the posterior palatine arches 

 and the uvula are so drawn together, as to prevent its passage into the 

 posterior nares. The larynx is drawn forwards beneath the root of 

 the tongue, and the epiglottis is pressed down over the rima glottidis, 

 so that nothing can enter the latter, unless drawn towards it by an 

 act of inspiration. When fairly within the pharynx, the alimentary 

 matter is seized by the constrictors which enclose that part of the 

 alimentary tube, and is drawn downwards by them into the oesopha- 

 gus, which is the cylindrical continuation of it. The continued action 

 of the constrictors serves to propel the food along the oesophagus ; their 

 movement being of a reflex nature, excited by the contact of the 

 substance contained in the tube, with its lining /membrane, which 

 produces an impression that is transmitted to the medulla oblongata, 

 and is reflected back as a motor impulse to the muscles. We have 

 here a distinct case of reflex action without sensation; for we have 

 no consciousness of the ordinary passage of food down the oesophagus, 

 unless it occasion pressure on the surrounding parts through its bulk, 

 or unduly irritate the lining membrane by its high or low temperature 

 or its acrid qualities ; and yet it may be shown by experiment, that tl 

 completeness of the nervous circle is requisite for the excitement of the 

 movement, which will not take place when it is interrupted either by 

 division of the nerves, or by destruction or paralysis of the medulla 

 oblongata. 



