NERVOUS MECHANISM. . 229 



fibres of the reflector epiglottidis and aryepiglotticus. (c) The closure of the glottis 

 by the constrictors of the larynx also prevents the entrance of substances into the 

 larynx (313, II. 2). 



Injury to the Epiglottis. Intentional injury of the epiglottis in animals, or its destruction 

 in man, may cause fluids to "go the wrong way," i.e., into the glottis, whilst solid food can be 

 swallowed without disturbance. In dogs, coloured fluids placed on the root of the tongue have 

 been observed to pass directly into the pharynx without coming into contact with it, so as to 

 tinge the upper surface of the epiglottis (Magendie). [The basis of the epiglottis is yellow 

 elastic cartilage, so that it shows no tendency to ossify, and always retains its elasticity ( 313).] 



In order that the descending bolus may be prevented from carrying the pharynx 

 with it, the stylo-pharyngeus, salpingo-pharyngeus, and baseo-pharyngeus contract 

 upwards when the constrictors act. 



Nervous Mechanism. Deglutition is voluntary only during the time the bolus 

 is in the mouth. When the food passes through the palatine arch into the gullet 

 the act becomes involuntary, and is, in fact, a well-regulated reflex action. When 

 there is no bolus to be swallowed, voluntary movements of , deglutition can be 

 accomplished only within the mouth ; the pharynx only takes up the movement, 

 provided a bolus (food or saliva) mechanically excites the reflex act. The afferent 

 nerves, which, when mechanically stimulated, excite the involuntary act of degluti- 

 tion, are, the palatine branches of the trigeminus (from the spheno-palatine gang- 

 lion) and the pharyngeal branches of the vagus. The centre for the nerves con- 

 cerned (for the striped muscles) lies in the superior olives of the medulla oblongata. 

 Swallowing can be carried out when a person is unconscious, or after destruction 

 of the cerebrum, cerebellum, and pons ( 367, 6). [Even in the deep coma of alco- 

 holism, the tube of a stomach-pump is carried into the stomach reflexly, provided 

 the surgeon passes it back into the pharynx.] The nerves of the pharynx are de- 

 rived from the pharyngeal plexus, which receives branches from the vagus, glosso- 

 pharyngeal, and sympathetic ( 352, 4). 



Within the oesophagus, whose stratified epithelium is moistened with the mucus 

 derived from the mucous glands in its walls, the downward movement is involun- 

 tary, and depends upon a complicated reflex movement discharged from the centre 

 for deglutition. There is a peristaltic movement of the outer longitudinal and 

 inner circular non-striped muscular fibres. 



In the upper part of the oesophagus, which contains striped muscular fibres, the peristalsis 

 takes place more quickly than in the lower part. The movements of the oesophagus never 

 occur independently, but are always the continuation of a foregoing act of deglutition. If food 

 be introduced into the oesophagus through a hole in its wall, there it lies ; and it is only carried 

 downwards when a movement to swallow is made. The peristalsis extends along the whole 

 length of the oesophagus, even when it is ligatured or when a part of it is removed {Mosso). If 

 a dog be allowed to swallow a piece of flesh tied to a string, so that the flesh goes halfway 

 down the oesophagus, and if the flesh be withdrawn, the peristalsis still passes downwards 

 (C. Ludwig and Wild). 



The motor nerve of the oesophagus is the vagus (not the accessory fibres) [oesophageal, whose 

 branches have numerous small ganglia in their course]. After it is divided, the food lodges in 

 the lower part of the oesophagus. Very large and very small masses are swallowed with more 

 difficulty than those of moderate size. Dogs can swallow an olive-shaped body weighted with a 

 counterpoise of 450 grammes (Mosso). When the thorax is greatly distended, as in Midler's 

 experiment, or greatly diminished, as in Valsalva's experiment ( 60), deglutition is rendered 

 more difficult. 



Goltz's Experiments. The oesophagus and stomach of' the frog become more excitable, i.e., 

 the excitability of the ganglionic plexuses in their walls is increased, when the brain and 

 spinal cord or both vagi are destroyed. These organs contract energetically after slight 

 stimulation, while frogs, whose central nervous system is intact, swallow fluids simply by 

 peristalsis. Females, and sometimes men also, suffering from hysteria, not unfrequently have 

 similar spasmodic contractions of the oesophageal region (globus hystericus). After section of 

 both vagi in the dog, Schiff observed spasmodic contraction of the oesophagus. 



Effect on Circulation. Every time one swallows, the heart's action is accelerated, the blood- 

 pressure falls, the necessity for respiration diminishes, while many movements (labour pains, 

 erection) are inhibited. These effects are brought about reflexly (Kronccker and Meltzer, 369). 



