464 THE CRANIAL NERVES. 



movements of the mouth and fauces, and by the continued action 

 of the upper part of the pharynx. 



It must be remembered that the general sensibility of the oeso- 

 phagus is very slight, as compared with that of the integument, or 

 even of the mucous membranes near the exterior. It is a general 

 rule, in fact, that the sensibility of the mucous membranes is most 

 acute at the external orifices of their canals ; as, for example, at the 

 lips, anterior nares, anus, orifice of the urethra, &c. It diminishes 

 constantly from without inward, and disappears altogether at a 

 certain distance from the surface. The sensibility of the pharynx 

 is less acute than that of the mouth, but is still sufficient to enable 

 us to perceive the contact of ordinary substances; while in the 

 oesophagus we are not usually sensible of the impression of the food 

 us it passes from above downward. The reflex action takes place 

 here without any assistance from the consciousness ; and it is only 

 when substances of an unusually pungent or irritating nature are 

 mingled with the food, that its passage through the oesophagus pro- 

 duces a distinct sensation. 



Larynx. We have already described the course and distribution 

 of the two laryngeal branches of the pneumogastric. The superior 

 laryngeal nerve is principally the sensitive nerve of the larynx. 

 Its division destroys sensibility in the mucous membrane of this 

 organ, but paralyzes only one of its muscles, viz : the crico-thyrpid. 

 Galvanization of this nerve has also been found to induce con- 

 tractions in the crico-thyroid, but in none of the other muscles 

 belonging to the larynx. The inferior laryngeal, on the other 

 hand, is a motor nerve. Its division paralyzes all the muscles of 

 the larynx except the crico-thyroid ; and irritation of its divided 

 extremity produces contraction in the same muscles. The muscles 

 and mucous membrane of the larynx are therefore supplied by two 

 different branches of the same trunk, viz., the superior laryngeal 

 nerve for the mucous membrane, and the inferior laryngeal nerve 

 for the muscles. 



The larynx, in man and in all the higher animals, performs a 

 double function ; one part of which is connected with the voice, the 

 other with respiration. 



The formation of the voice in the larynx takes place as follows. 

 If the glottis be exposed in the living animal, by opening the 

 pharynx and oesophagus on one side, and turning the larynx for- 

 ward, it will be seen that so long as the vocal chords preserve 

 their usual relaxed condition during expiration, no sound is heard, 



