CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 483 



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 spinal bulb. De- 

 glutition can be excited, by tickling the fauces, in an animal 

 rendered unconscious by removal of the brain, provided the 

 spinal bulb be left. If the bulb 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 spinal bulb 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. 



270. Movements of the (Esophagus. These are, so far as the 

 mere muscular act is concerned, peristaltic in nature. The circular 

 contraction begun by the constrictors of the pharynx is continued 

 along the circular coat of the oesophagus and is assisted by an 

 accompanying contraction of the longitudinal coat, the direction 

 being always, save in the abnormal action of vomiting, from above 

 downwards. 



It will be remembered ( 222) 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, so 

 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. 



These peristaltic movements of the oesophagus may, like those 

 of the intestine, be seen after removal of the organ from the body ; 

 and indeed may continue to appear, upon stimulation, for an 

 unusual length of time. They may therefore be carried out by 

 the muscular elements, with or without the help of the nervous 

 elements embedded in them, apart from any action of the central 

 nervous system. Nevertheless, in the living body, the movements 

 of the oesophagus seetn to be in a special way dependent on the 

 central nervous system; the contractions are not started and 

 carried out by the walls of the tube alone and so transmitted from 

 section to section in the walls of the tube itself; but afferent 

 impulses started in the pharynx and passing to the spinal bulb, 

 give rise to reflex efferent impulses which descend along nervous 

 tracts to successive portions of the organ. If the oesophagus be 

 cut across some way down, or if a portion of the middle region 

 be excised, stimulation of the pharynx will produce a peristaltic 



