3 2o MECHANISMS OF THE DIGESTIVE TRACT. 



itself, or by forcing a certain amount of saliva into the fauces. The 

 afferent impulses for this reflex act travel by branches of the second 

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

 pharyngeal branches of the superior laryngeal nerve. By excitation of 

 the latter nerve, it is possible to evoke either a single act or a whole 

 series of acts of deglutition. 



The centre of deglutition lies in the medulla oblongata, in the 

 neighbourhood of the nucleus of the vagus nerve. According to 

 Marckwald, 1 it can be destroyed without injuring the adjacent re- 

 spiratory centre. After such destruction, stimulation of the superior 

 laryngeal nerve causes total relaxation of the diaphragm, unaccompanied 

 by the small twitches (respirations of swallowing) which accompany or 

 precede the normal acts of deglutition. 



The efferent impulses from the centre pass by the hypoglossal 

 nerve to the muscles of the tongue, by the fifth to the mylohyoid 

 muscle, by the glosso-pharyngeal, the pharyngeal branches of the vagus, 

 the fifth, and the spinal accessory nerves, to the muscles of the fauces 

 and pharynx. The closure of the larynx is effected by impulses which 

 travel through the superior and inferior laryngeal branches of the 

 vagus. 



The distribution of nerves to the oesophagus varies in different 

 animals. In man the upper part of the oesophagus receives branches from 

 the recurrent laryngeals, the lower part from the pulmonary and 

 oesophageal plexuses of the vagus. In the rabbit and also in the 

 guinea-pig some of the fibres to the upper portion of the oesophagus 

 run in a distinct nerve, which rises from the trunk of the vagus and 

 passes upward to its destination. 2 In the horse and dog the inner- 

 vation of the lower segment of the oesophagus resembles that in man. 

 Most of the fibres to the upper segment are derived, however, from a 

 small branch of the pharyngeal nerve, which travels alongside of the 

 oesophagus down into the chest, sending numerous branches into the 

 muscular wall on its way. 



The effects of section of the oesophageal nerves vary in the different 

 segments of this tube. Whereas the upper part becomes permanently 

 relaxed, the lower segment, including the cardiac sphincter, enters into 

 a condition of tonic contraction, which may last several days. These 

 results suggest that, although the contraction of the oesophagus 

 resembles in many particulars the autocthonous, peristaltic contractions 

 of other muscular tubes, such as the intestine and ureter, it will 

 probably differ from these in being dependent on nervous influences 

 from outside. As a matter of fact, Mosso 3 has shown that the pro- 

 pagation of contraction down the oesophagus is a reflex act, and requires 

 the integrity of the nervous connections of the oesophagus with the 

 centre of deglutition. If the oesophagus be ligatured or cut across, 

 waves of contraction, originated from the pharynx, pass across the 

 lesion from one separated end to the other, as before. On the other 

 hand, section of the nerves going to the oesophagus abolishes its peri- 

 staltic contractions. The peristaltic contraction of the oesophagus, 

 therefore, like that of the pharynx, is a reflex started from the stimula- 



1 "The Movements of Respiration," trans, by Haig, London, 1888, p. 134. 



2 Steiner, Verhandl. d. natvrh.-mcd. Vcr. zu Heidelberg, 1879, N. F., Bd. ii. S. 183 

 (quoted by S. Mayer, Hermann's "Handbuch," Bd. v. S. 424). 



3 Unlersuch. z. Naturl. d. Mensch. u. d. Thlcre, 1876, Bd. xi. S. 331. 



