CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 455 



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 

 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 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. 



269. Peristaltic movements. Putting aside the somewhat 

 complicated pharyngeal part of deglutition, and taking the ceso- 

 phageal movements by themselves we find that these, together 

 with the movements of the stomach, and of the small and large 

 intestines right down to the anus are more or less alike, and may 

 be described under the general name of ' peristaltic ' movements. 

 We have already in 92, spoken of these, but it may be well to 

 consider them briefly again under a general aspect, before dwelling 



