160 A MANUAL OF VETERINARY PHYSIOLOGY 



even bulky material can pass along it ; what the ox and dog can 

 swallow with ease would certainly ' choke ' the horse. 



It is now believed that in man liquid or liquid foods are held 

 up at the cardiac end of the oesophagus and slowly pass into the 

 stomach, the opening into which is controlled by a sphincter. 

 We have never observed anything in the horse which would 

 lead us to assume a similar action. The cardiac sphincter in 

 this animal is very powerful, and extends some inches up the 

 oesophagus ; food may sometimes be found in it which has failed 

 to obtain an entry to the stomach, but it is always trifling in 

 amount. 



The first stage of deglutition is voluntary, but the remaining 

 processes are quite involuntary, and are brought about by the 

 stimulation of a centre in the medulla known as the swallowing 

 centre. By means of ingoing or afferent nerves supplied by 

 branches of the fifth and the superior laryngeal, the centre is 

 made acquainted with the fact that food is present in the fauces. 

 A reflex act is now set up in the centre, and an impulse conveyed 

 to the muscles of the part by outgoing or efferent nerves, furnished 

 by the pharyngeal plexus (composed of the vagus and glosso- 

 pharyngeal) to the constrictor muscles of the pharynx, by the 

 hypoglossal to the tongue, and by the recurrent laryngeal to 

 the muscles which close the glottis. The glosso-pharyngeal is 

 the inhibitory nerve of deglutition ; if the central end be stimu- 

 lated it is impossible to produce the act of swallowing. Further, 

 it is the nerve which immediately inhibits respiration during 

 swallowing, no matter at what phase of the act — viz., inspiration 

 or expiration — the stimulus is applied. Swallowing may be 

 induced without the presence of food in the fauces ; touching 

 the rim of the glottis will produce it ; so also will pouring a fine 

 jet of fluid into the trachea, or even touching the interior of the 

 trachea as far down as the bronchi. 



Stimulation of the mucous membrane of the pharynx excites 

 reflex movements of the oesophagus, but stimulation of the mucous 

 membrane of the oesophagus itself is ineffective in this respect. 



The swallowing centre also presides over the oesophagus, and 

 the peristaltic wave from the pharynx to the stomach is produced 

 by impulses sent out from this centre through the vagus. This 

 wave is, therefore, not due to the nerve handing on a contraction 

 by direct conduction from one layer of the muscular wall of the 

 oesophagus to the next. Hence, when once started, it is not 

 arrested either by ligaturing or dividing the oesophagus, though 

 section of the oesophageal nerves prevents it. The contraction 

 wave which sweeps along the oesophagus is not interfered with 

 even by excising a portion of the tube ; the wave, having reached 

 the point from which the upper segment has been cut out, appears 



