296 THE TISSUES AND MECHANISMS OF DIGESTION. 



The first stage in this complicated process is undoubtedly a voluntary 

 act. The raising of the soft palate and the approximation^ the poste- 

 rior 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 and in the absence of consciousness. 

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

 component movements are, as it were, on the borderland between the volun- 

 tary and involuntary kingdoms, must be regarded as a reflex act. The 

 third and last stage, whatever be the exact form which it takes, is undoubt- 

 edly reflex ; the will has no power whatever over it, and can neither origi- 

 nate, stop, nor modifiy 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 movements 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 about by centripetal stimulation of the superior laryn- 

 geal 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 different 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 the 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. 



231. Movements of the oesophagus. 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 downward. 



It will be remembered that the muscular bundles of the oesophagus are 

 composed of striated fibres in the upper part, and of plain unstriated fibre 

 cells in the lower part, the transition occupying a different level in different 

 animals. Nevertheless, as far as the peristaltic movement is concerned, the 

 two kinds of fibres behave in the same way, except that the peristaltic wave, 

 if we may so call it, travels more rapidly in the striated region. 



