CHAP. VIL] SPECIAL MUSCULAR MECHANISMS. 1465 



by the extrinsic laryngeal muscles, such as the thyro-hyoid, sterno- 

 thyroid, pharyngeal muscles and others. The exact position in 

 which it is fixed will depend on the pitch of the notes which are 

 uttered ; it is raised for high notes and lowered for low ones, and 

 may be fixed either above or below or at the natural position of 

 rest. 



We are accustomed to classify voices according to the range of 

 pitch within which the voice can sing truly and with ease, and we 

 thus distinguish, in ascending scale, such voices as bass, barytone 

 and tenor in the male, alto, mezzo-soprano and soprano in the 

 female. Could we consider the vocal cord as a membranous 

 edge, possessing a form and nature which was constant or varied 

 only with age, so that the rapidity of its vibrations, and hence the 

 pitch of the voice, depended solely upon its length, fixed by the 

 growth of the individual, and upon its varying tension, determined 

 by muscular contraction, the result being influenced by the varying 

 width of the glottis, the structural basis of the distinction between 

 the several kinds of voice would be simple enough ; the bass and the 

 contralto voices would have long vocal cords, and the other voices 

 in each sex would be in ascending scale successively shorter. The 

 vocal cord, however, is not of such a permanent nature; it undergoes 

 under the influence of muscular contraction changes other than 

 those of tension affecting its whole length ; its form may be altered 

 and the positions or attitudes which it may assume cannot be 

 described as simply those of greater or less distance from its 

 fellow along its whole length. It is in section as we have said 

 wedge-shaped ; and the projecting angle of the wedge may be 

 an open broad one, or a narrow acute one ; the vibrating cord 

 may be thick or thin; and its vibrations will vary accordingly. 



The change from thick to thin is apparently brought about by 

 muscular contraction; it has been suggested that a partial con- 

 traction of some of the fibres of the thyro-arytenoid muscle, 

 external and also internal, more particularly of the bundles 

 which take a more or less vertical direction, produce the result ; 

 but the exact mechanism is by no means clear, though special 

 examination of the larynx shews that such a change of thickness 

 may take place. 



Again, there are reasons for thinking that contraction of the 

 internal thyro-arytenoid muscle as a whole affects the form and 

 the physical condition of the vocal cord, of which it furnishes so 

 to speak the body; the strand of elastic fibres which forms the 

 surface of the cord lies upon the muscle somewhat after the 

 fashion of a fascia, and when the muscle, which in a state of 

 rest is somewhat curved with the concavity towards the glottis, 

 thickens and shortens in its contraction, carrying with it the 

 overlying layer of elastic fibres, it brings the whole cord into a 

 different form and different physical condition ; and this must 

 affect the character of the vibrations. Again, it is maintained 



