PNEUMOGASTRIC, OR PAE VAGUM NERVE. 651 



accessory. Their direct action upon the muscles of deglutition belongs to the physiologi- 

 cal history of the last-named nerve. We have already stated, in treating of the spinal 

 accessory, that the filaments of communication that go to the pharyngeal branches of the 

 pneumogastric are distributed to the pharyngeal muscles. 



It is impossible to divide all of the pharyngeal filaments in living animals and observe 

 directly how far the general sensibility of the pharynx and the reflex phenomena of 

 deglutition are influenced by this section. As far as we can judge from the distribution 

 of the filaments to the mucous membrane, it would seem that they combine with the 

 pharyngeal filaments of the fifth, and possibly with sensory filaments from the glosso- 

 pharyngeal, in giving general sensibility to these parts. 



In some recent experiments by Waller and Prevost, upon the reflex phenomena of 

 deglutition, it is shown that the action of the pharyngeal muscles cannot be excited by 

 stimulation of the mucous membrane of the supra-laryngeal region and the pharynx, after 

 section of the fifth and of the superior laryngeal branch of the pneumogastric. This 

 would seem to show that the pharyngeal branches of the pneumogastrics are of little or 

 no importance in these reflex phenomena. 



Properties and Functions of the Superior Laryngeal Nerves. The distribution of 

 these nerves points to a double function ; viz., an action upon the crico-thyroid muscles, 

 and the important office of supplying general sensibility to the upper part of the larynx 

 and a portion of the surrounding mucous membrane. The stimulation of these nerves' 

 produces intense pain and contraction of the crico-thyroids ; but it has been shown 

 by experiment that the arytenoid muscles, through which the nerves pass, receive no 

 motor filaments. The action of the nerves upon the muscles is very simple, and resolves 

 itself into the function of the crico-thyroids, which has been treated of fully under the 

 head of phonation. When these muscles are paralyzed, the voice becomes hoarse. The 

 filaments to the inferior muscles of the pharynx are few and comparatively unimportant. 

 It is important in this connection to note that the superior laryngeals do not receive their 

 motor filaments from the spinal accessory. 



The sensory filaments of the superior laryngeals have important functions connected 

 with the protection of the air-passages from the entrance of foreign matters, particularly 

 in deglutition, and are farther concerned, as we shall see, in the reflex action of the con- 

 strictors of the pharynx. In treating of deglutition, we have fully discussed the impor- 

 tance of the exquisite sensibility of the top of the larynx in the protection of the air- 

 passages. When both superior laryngeals have been divided in living animals, liquids 

 often pass into the larynx in small quantity, owing to the absence of the reflex closure 

 of the glottis when foreign matters are brought in contact with its superior surface, and 

 the occasional occurrence of inspiration during deglutition. 



Aside from the protection of the air-passages, the superior laryngeal is one of the 

 sensory nerves through which the reflex acts in deglutition operate. There are certain 

 parts which depend for their sensibility entirely upon this nerve ; viz., the mucous mem- 

 brane of the epiglottis, the aryteno-epiglottidean fold, and the larynx as far down as 

 the true vocal cords. When an impression is made upon these parts, as when they 

 are touched with a piece of meat, regular and natural movements of deglutition ensue. 



The experiments made by galvanizing the trunks of the superior laryngeal nerves are 

 extremely interesting. If the nerves be divided and galvanization be applied to their 

 central ends, movements of deglutition are observed, and there is also arrest of the 

 action of the diaphragm. From these experiments, it would seem that the impression 

 which gives rise to the movements of deglutition aids in protecting the air-passages 

 from the entrance of foreign matters, by temporarily arresting the inspiratory act. 



An important point for our consideration, in this connection, is the action of the 

 superior laryngeal nerves in the ordinary phenomena of deglutition ; and, in experiments 

 with galvanism, a feeble current simulates most nearly the natural processes. In such 



