Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 375 



cise of the will and in the absence of consciousness. 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, caused in the ordinary way by 

 the food coming in contact with the fauces, the afferent impulses 

 originated in the fauces are carried up to the nervous centre by 

 the glosso-pharyngeal nerve, by branches of the fifth, and by the 

 pharyngeal branches of the superior laryngeal division of the 

 vagus. The latter seem of special importance, since the act of 

 swallowing, quite apart from the presence of food in the mouth, 

 may be brought out by centripetal stimulation of the superior 

 laryngeal nerve. The efferent impulses descend the hypoglossal 

 to the muscles of the tongue, and pass down the glosso-pharyngeal, 

 the vagus through the pharyngeal plexus, the fifth, and the spinal 

 accessory, to the muscles of the fauces and pharynx : their exact 

 paths being as yet not fully known, and probably varying in differ- 

 ent animals. The laryngeal muscles are governed by the laryngeal 

 branches of the vagus. 



The centre of the reflex act lies in the medulla oblongata. 

 Deglutition can be excited, by tickling . the fauces, in an animal 

 rendered unconscious by removal of the brain, provided the 

 medulla be left. If the medulla be destroyed, deglutition is 

 impossible. The centre for deglutition lies higher up than that 

 of respiration, so that in diseases or injuries involving the upper 

 part of the medulla oblongata the former act may be impaired or 

 rendered impossible while the latter remains untouched. It has 

 been said to form part of the superior olivary bodies, but this view 

 is based on anatomical grounds only. We shall have to deal with 

 this and similar matters in treating of the central nervous system. 

 It is probable that, as is the case in so many other reflex acts, the 

 whole movement can be called forth by stimuli affecting the centre 

 directly, and not acting on the usual afferent nerves. 



§ 220. Movements of the (Esophagtis. These as we have just 

 said are fairly simple. The circular contraction begun by the 

 constrictors of the pharynx is continued along the circular coat of 

 the oesophagus and assisted by an accompanying contraction of the 

 longitudinal coat, the direction being always, save in the abnormal 

 action of vomiting, from above downwards. 



