548 VOICE AND SPEECH 



particular branches which govern the function of this organ are the 

 superior and inferior laryngeal nerves (Fig. 263). In general it may 

 be said that their innervation is unilateral in character, but a slight 

 median overlapping, especially with regard to the sensory fibers, is 

 not uncommon. It has been shown above that the superior branches 

 are motor as well as sensory in their function, while the inferior or 

 recurrent branches are wholly motor. The motor qualities of the 

 former are restricted to their rami externi which supply the crico- 

 thyroid muscles. These muscles, as we have just seen, govern the 

 vertical approximation of the thyroid and cricoid cartilages and deter- 

 mine, therefore, the tension of the vocal cords. Consequently, it may 

 be stated that the inferior branches control all the muscles of the larynx 

 with the exception of the cricothyroids. 



Keeping these facts clearly in mind, it must be evident that the 

 stimulation of the intact superior laryngeal nerve, or of the distal 

 end of the divided nerve, leads to an approachment of the thyroid and 

 cricoid cartilages and an increased tension of the vocal cords. The 

 glottis is slightly narrowed by this action, owing to the fact that the 

 arytenoid cartilages are not sufficiently resistant to withstand the pull 

 exerted by the vocal cords. The cricothyroid muscle as such, however, 

 does not serve as an adductor of the vocal cords. As has been stated 

 in one of the preceding paragraphs, the sensory qualities of this nerve 

 may be ascertained by the stimulation of the intact nerve or of its 

 central end. With currents of moderate strength, this procedure 

 evokes a respiratory standstill and forced expiratory blasts. 



Certain evidence has been presented to show that the inferior 

 laryngeal nerve of the apes also conducts in an afferent direction. This 

 is also true of the corresponding nerve in the dog and cat, but only 

 under special conditions. In view of this uncertainty, it seems best 

 to regard this nerve essentially as a motor path for those impulses 

 which give rise to the different sphincter actions of the larynx, and 

 especially to that occurring at the level of the vocal cords. Attention 

 should also be called to the fact that the vagus innervates extensive 

 segments of the pharynx and esophagus and is thus placed in a position 

 to correlate the action of the laryngeal musculature with that of the 

 muscles used during the process of deglutition. 1 



In accordance with these statements, it may be concluded that 

 the division of either inferior laryngeal nerve must lead to a paralysis 

 of the muscles on the corresponding side of the larynx, excepting, of 

 course, the cricothyroid muscle. Quite similarly, the division of 

 both nerves must result in a bilateral paralysis, the aforesaid muscles 

 being excepted. In young animals, this procedure is usually followed 

 by serious symptoms, death from asphyxia resulting in the course 

 of a few days. But, while it is true that the vocal cords assume an 

 extreme median position in consequence of the paralysis of the aryte- 

 noid muscles, this condition cannot be regarded as the sole cause of 



1 Schultz and Dorendorf, Archiv fur Laryngologie, xv, 1904. 



