1212 



ON VOCAL SOUNDS. 



when it is placed over the thyro-cricoid membrane. This relaxation of the 

 thyro-cricoid muscle is accompanied by contraction of the thyro-arytenoid, 

 with the result that the cord is rendered narrower, and now only those 

 parts lying near the edge vibrate. The tension of the cords is still main- 

 tained by the posterior fibres of the thyro-cricoid and by the thyro-hyoid. 1 



According to Bosworth, the sense of relief in passing from the lower to 

 the middle register is due for the most part to the fact that less muscular 

 effort is required to throw the narrow portion of the cord into vibration, than 

 was necessary in the lower register to set the cord into vibration in its whole 

 breadth. The natural result of this diminution of the vibrating mass is that 

 the voice loses considerably in volume ; at the same time, however, it gains in 

 softness. In this register the larynx appears to become narrower than in the 

 lower, and simultaneously with this narrowing the ventricular bands approach 

 one another. It is quite possible that the diminution in the breadth of the 

 vocal cords themselves may be in large part merely apparent, their outer 

 portion being hidden by the ventricular bands, so that they may be really as 

 broad as in the lower register. 



The rise in pitch which occurs during the transition from the lower to 

 the middle register is, however, undoubtedly due in part to a thinning of 

 the vocal cords. This is best seen by using Yoltolini's method of illuminating 

 the larynx, that is, by placing a strong light directly in front of the thyroid 

 cartilage, instead of reflecting it through the mouth and pharynx. When the 

 mirror is put into the throat, the cords now appear pink or red in colour, and 

 changes in their thickness can easily be observed. In the middle register they 

 are beautifully transparent, but much less so in the lower. 



On ascending in the middle register, the cords gradually become more 

 and more tense as the anterior fibres of the thyro-cricoid muscle again 

 come into play, and the crico- thyroid space becomes obliterated. The 

 pitch can be raised no further by this mechanism. It must be added 

 that, in the whole of the middle register, the vocal processes of the 

 arytenoid cartilages are kept in contact by the action of the lateral 

 crico-arytenoid and the inter-arytenoid muscles. 



Head register. Having reached the highest note which can be 



produced in the middle register, the 

 mechanism must now be changed again, 

 if a series of higher notes is required. 

 This new mechanism is called the 

 head register. By far the most striking 

 change that takes place is, that the 

 vocal cords are now brought closely in 

 contact along their posterior halves, 

 while there remains an oval orifice 

 at the front. Thus the only vibrating 

 portions of the cord are those which 

 bound this opening (Fig. 429). 



The tension of the vocal cords in 

 tn i g register is still maintained by 



production of a note in the head the thyro-cricoid and thyro-arytenoid 



muscles. As the pitch rises the small 

 orifice in front becomes shorter and narrower, until at the highest notes 

 only the anterior third of the cords may be vibrating. 



In the preceding description of the registers, that classification has 



1 Bosworth, "Diseases of the Throat and Nose," vol. ii. p. 469. 



FIG. 429. Aperture of glottis during 



