in PHONATION AND ARTICULATION 141 



the muscles of the larynx, so that the vocal cords assume the 

 position of elastic equilibrium as in the dead body. Under 

 these conditions the glottis is moderately open, in the form of 

 mi isosceles triangle, with the angle of the apex towards the 

 attachments of the vocal cords on the inner surface of the thyroid. 

 Contraction of the laryngeal muscles is therefore not required 

 to hold the glottis open, as it must be in respiration. Laryngo- 

 scopic observations show, however, that during quiet respiration 

 when no voluntary influence is exerted upon the laryngeal 

 muscles the glottis is more widely open than after death. In 

 quiet respiration the glottis has an average width of 14 mm. in 

 the adult man, and about 11 mm. in a woman, while on the dead 

 subject it is about 5 mm. and 4 mm. respectively. This striking 

 difference shows that in life the posterior crico-arytenoid muscle 

 is kept continuously in a state of semi-contraction by the reflex 

 or automatic tonic activity of a centre, which acts exclusively 

 or predominatingly upon those fibres of the recurrens which 

 innervate the abductors of the vocal cords. 



In many animals this tonic contraction of the abductors of 

 the glottis varies with the rhythm of the respiratory muscles ; 

 at each inspiration the glottis dilates, and at each expiration 

 it is slightly constricted. In man, however, laryngoscopical 

 observation shows that during quiet breathing these respiratory 

 oscillations of the glottis do not occur in the great majority of 

 cases (Sernon), and only appear during forced or dyspnoeic 

 respiration (see Vol. I. p. 421). 



After section of the recurrent laryngeal nerve this respiratory 

 rhythm ceases, and the cords take up the paralytic position of 

 moderate separation which is seen after death. 



Section of one recurrent nerve alone deforms the glottis owing 

 to disappearance of the tone of the muscles on the paralysed side, 

 which brings the vocal cord of that side nearer the median line. 

 This deformation or asymmetry of the glottis increases during 

 forced respiration. 



The most important effect of section of the recurrent nerves is 

 the aphonia first described by Galen. Total loss of the voice is 

 not, however, constant. Haller, J. Miiller, Magendie, and others 

 noted that many dogs continue to bark after section of the 

 recurrent nerves, while others are still capable of emitting high 

 notes, especially when suffering acute pain. Longet confirmed this 

 fact, and found that the power of uttering high sounds was ob- 

 served only in dogs a few months old, in which the tension of the 

 vocal cords produced by the action of the crico-thyroid muscles, 

 which are not paralysed by section of the recurrent nerves, 

 suffices for the formation of high sounds, the inter-cartilaginous 



O t O 



portion of the glottis not being fully developed, owing to the almost 

 total absence of the vocal processes, so that the cords are kept 



