568 THE CRANIAL NERVES. 



as an "action of arrest." The same nervous impression which excites 

 by reflex action the constrictors of the pharynx, suspends for a time 

 the movements of inspiration. This effect is very perceptible in the 

 ordinary act of swallowing, and was witnessed by Waller and Prevost in 

 many of their experiments on this subject ; galvanization of the central 

 extremity of the superior laryngeal nerve causing immediate relaxation 

 of the diaphragm, with stoppage of its movements. 



The effect of the arrest of breathing upon the glottis is to prevent the 

 customary opening of its orifice at the time of inspiration. As the res- 

 piratory movements of the glottis are coincident with those of the chest, 

 and are excited and maintained by the same nervous influence, the 

 impression which puts a stop to one suspends the other also. The glot- 

 tis consequently, not being opened at the time the food enters the 

 pharynx, its liability to admit any portion of the alimentary mass is 

 much diminished by the mere fact of its passive condition. But this 

 condition furthermore allows the rima glottidis to be completely closed 

 by the contraction of the inferior constrictor of the pharynx, the most 

 active muscle in the apparatus of deglutition ; since the fibres of this 

 muscle are attached laterally to the external surface and free borders 

 of the thyroid cartilage, and thus compress the larynx on both sides at 

 the moment the food is carried downward by their contraction. It is 

 by this means alone that the glottis is protected in birds and in other 

 animals where the epiglottis is wanting, and it is also the essential part 

 of the same process in man and in mammalians. 



The accident in which food or foreign substances sometimes gain 

 access to the larynx is always produced by a sudden attempt at in- 

 spiration. This, which cannot take place during deglutition in the 

 ordinary condition of the nervous system, may nevertheless be produced 

 in many instances by an unexpected shock or excitement, which disturbs 

 momentarily the harmonious -co-ordination of the reflex actions. Any 

 sudden impression produces in general, as its first effect, a spasmodic 

 movement of inspiration ; and if this take place while food is contained 

 in the pharynx, a portion of it almost necessarily passes in, together 

 with the current of air, through the widely open orifice of the glottis. 



Connection with the Stomach and Stomach Digestion. The effect 

 produced upon the stomach and digestion by division of the pneumo- 

 gastric nerve shows that its influence upon this organ is in the main 

 similar to that which it exerts on the oesophagus ; that is, it confers on 

 its mucous membrane a certain sensibility to the presence of food, and 

 provides for the peristaltic action of its muscular coat. After experi- 

 mental section of both pneumogastric nerves in the region of the neck, 

 the sensations of hunger and thirst remain ; the animals often exhibiting 

 a desire for food and drink, and sometimes taking it in considerable 

 quantity, although little, if any, reaches the stomach, owing to the 

 paralysis of the muscular walls 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 



