140 



PHYSIOLOGY 



CHAP. 



phonation with other laryngeal muscles. If we assume that 

 during contractions of the muscles which stretch the vocal cords, 

 the internal thyro-arytenoids which tend to relax them are also 

 contracting, it is easy to understand the functions of the latter, 

 which regulate the delicate changes in position of the larynx 

 and vocal cords necessary in a gradual succession of tones that 

 differ little in strength and pitch from each other. The feeling 

 of tension in the larynx in singing with the chest register fully 



open shows that in singing all 

 , .1 -v > the laryngeal muscles may be 



, Ycfv more or less active, and that the 



formation of different musical 

 notes, gradations of their pitch, 

 and rise and fall in the scale, 

 depend on the delicate co-ordina- 

 tions of their activity, and par- 

 ticularly on the internal thyro- 

 arytenoids, which are in direct 

 and intimate relation with the 

 vibrating vocal cords, and have 

 justly been named the "vocal 

 muscles." 



III. The nerves to the larynx 

 are the two laryngeal branches 

 of the vagus (Fig. 96). The 

 superior laryngeal certainly con- 

 tains more sensory than ; motor 

 fibres ; the former are distributed 

 by the ramus internus to the 

 mucous membrane of the larynx 



FIG. 96. Laryngeal nerves from behind, and tO the laryngeal muscles aS 



SBE^tafl^^TttTafi fibres of muscular sense; the 



sswsarasssfssai motor fibres p ass .&*<>*& ^ 



nerves ; 7, same on the right : 8, 8, inferior ramUS CXtemUS to innervate the 

 laryngeal nerves ; !>, branches to posterior , i i i , i i 



crico- arytenoid muscles; 10, branch to CriCO-thyrOlQ mUSCleS, partly alSO 



arytenoid muscle ; 11, 12, branches to crico- r V, p orvfpnnirl mnjplp 

 arytenoid and thyro-arytenoid muscles. ctij UolJ >iu muou.it;. 



The inferior laryngeal, or 



nervus recurrens, is a purely motor branch which supplies all 

 the muscles of the larynx except the crico-thyroid. 



As Claude Bernard observed complete aphonia in cats after 

 extirpation of the spinal accessory, it was generally held that 

 the motor fibres of the larynx came from the ramus internus 

 (accessorius vagi) of this nerve, although they ran in the vagus. 

 But the later work of Grabower (1890) showed that the motor 

 branches to the larynx originate in the vagus, and more 

 particularly from its lower roots. 



Section of both laryngeal nerves produces relaxation of all 



