488 THE NERVOUS SYSTEM. 



glottis against the entrance of food does not depend on a mechani- 

 cal obstacle, but on a special association of nervous acts. 



The first requisite for swallowing is the suspension *of respiration. 

 This takes place, at the beginning of deglutition, by an influence desig- 

 nated as the " action of arrest." The same nervous impression which 

 excites contraction of the pharynx, suspends for a time the movement 

 of inspiration. 



The effect of this arrest is to prevent the opening of the glottis. 

 As the respiratory movements of the glottis are coincident with those 

 of the chest, and are excited by the same nervous influence, the impres- 

 sion which puts a stop to one also suspends the other. The glottis 

 consequently not being opened when food enters the pharynx, its 

 liability to admit any portion of the alimentary mass is considerably 

 diminished. But it is furthermore completely closed by the inferior 

 constrictor of the pharynx, the most active muscle in the apparatus of 

 deglutition ; since the fibres of this muscle are attached to the exter- 

 nal surface and borders of the thyroid cartilage, thus compressing the 

 larynx on both sides at the instant of deglutition. By this means the 

 glottis is protected, as in birds and reptiles, even where an epiglottis is 

 wanting. 



The accident by which food or foreign substances sometimes gain 

 access to the larynx, in man, is always caused by a sudden attempt at 

 inspiration. This cannot take place during deglutition in the ordinary 

 state of the nervous system ; but it may be produced by any unex- 

 pected shock or excitement which disturbs the coordination of the 

 reflex actions. Such a shock usually causes, as its first effect, a spas- 

 modic inspiration ; and if this take place while food is passing through 

 the pharynx, a portion of it finds its way through the open orifice of 

 the glottis into the larynx. 



Connection with Stomach Digestion. The effect produced on the 

 stomach by division of the pneumogastric nerve shows that its influ- 

 ence on this organ is mainly similar to that which it exerts on the 

 oesophagus ; that is, it supplies the mucous membrane with a special 

 sensibility to the contact of food, and provides for the peristaltic action 

 of the muscular coat. After section of both pneumogastric nerves in 

 the neck, the sensations of hunger and thirst remain; the animals 

 often exhibiting a desire for food and drink, which they sometimes take 

 in considerable quantity, though but little reaches the stomach, owing 

 to the paralysis of the oesophagus. In the experiments of Bernard on 

 dogs, the secretion of gastric juice was suspended after this operation, 

 and food introduced into the stomach through a gastric fistula remained 

 undigested. But Longet found that if the food were introduced only 

 in small quantity, it might cause the secretion of gastric juice, and be 

 finally digested. This indicates that secretion and digestion in the 

 stomach are not immediately under the control of the pneumogastric 

 nerve, but that after its section they become practically suspended, 

 owing mainly to paralysis of the muscular coat. 



