466 THE CRANIAL NERVES. 



by Bischoff and by Bernard 1 that if all the roots of the spinal acces- 

 sory be divided at their origin, or if the nerve itself be torn away 

 at its exit from the skull, all the other cranial nerves remaining 

 untouched, the voice is lost as completely as if the inferior laryn- 

 geal itself had been destroyed. All the motor fibres of the pneu- 

 mogastric, therefore, which act in the formation of the voice are 

 derived, by inosculation, from the spinal accessory nerve. 



In respiration, again, the larynx performs another and still more 

 important function. In the first place, it stands as a sort of guard, 

 or sentinel, at the entrance of the respiratory passages, to prevent 

 the intrusion of foreign substances. If a crumb of bread accidentally 

 fall within the aryteno-epiglottidean folds, or upon the edges of the 

 vocal chords, or upon the posterior surface of the epiglottis, the 

 sensibility of these parts immediately excites a violent expulsive 

 cough, by which the foreign body is dislodged. The impression 

 received and conveyed inward by the sensitive fibres of the superior 

 laryngeal nerve, is reflected back upon the expiratory muscles 

 of the chest and abdomen, by which the instinctive movements of 

 coughing are accomplished. Touching the above parts with the 

 point of a needle, or pinching them with the blades of a forceps, 

 will produce the same effect. This reaction is essentially dependent 

 on the sensibility of the laryngeal mucous membrane ; and it can 

 no longer be produced after section of the pneumogastric nerve, or 

 of its superior laryngeal branch. 



In the second place, the respiratory movements of the glottis, already 

 described in a previous chapter, are of the greatest importance to 

 the preservation of life. We have seen that at the moment of 

 inspiration the vocal chords are separated from each other, and the 

 glottis opened, by the action of the posterior crico-arytenoid muscles ; 

 and that in expiration the muscles and the vocal chords are both 

 relaxed, and the air allowed to pass out readily through the glottis. 

 The opening of the glottis in inspiration, therefore, is an active 

 movement, while its partial closure or collapse in expiration is a 

 passive one. Furthermore, the opening of the glottis in inspiration 

 is necessary in order to afford a sufficiently wide passage for the 

 air, in its way to the trachea, bronchi, and pulmonary vesicles. 



Now we have found, as Budge and Longet had previously no- 

 ticed, that if the inferior laryngeal nerve on the right side be 

 divided while the glottis is exposed as above, the respiratory move- 



1 Recherches Experimentales sur les fonetions dn nerf spinal. Paris, 1851. 



