520 ACTIONS OF THE LARNYX *. —VOICE. 
larynx sets them in vibration, in a manner very much resem¬ 
bling that in which the reed of a Hautboy or Clarionet, or the 
tongue of an Accordion or Harmoninm, is set in vibration by 
the current of air that is made to pass beneath them. The 
rapidity of the vibrations, and consequently the pitch of the 
sound (§ 523), depends on the degree of tension or tightness 
of the vocal ligaments; and this is regulated by muscles which 
act upon the thyroid and arytenoid cartilages. If the thyroid 
cartilage he drawn forwards, and the arytenoid cartilages back¬ 
wards, the two ends of the vocal cords will he further sepa¬ 
rated from each other, and they will consequently be tightened; 
by the contrary movements they will he relaxed. 
683. It is on account of the greater length of the vocal 
cords, that the pitch of the voice is much lower in Man than 
in Woman; hut this difference does not arise until the end 
of the period of childhood, the size of the larynx being about 
the same in the Boy and Girl up to the age of 14 or 15 years, 
but then undergoing a rapid increase in the former, whilst it 
remains nearly stationary in the latter. Hence it is that Boys, 
as well as Girls and Women, sing treble; whilst Men sing 
tenor which is about an octave lower than the treble, or bass 
which is lower still. 
684. The cause of the variations of timbre or quality in 
different voices, is not certainly known; but it appears to be 
due, in part, to differences in the degree of flexibility and 
smoothness in the cartilages of the larynx. In women and 
children these cartilages are usually soft and flexible, and 
their voices clear and smooth ; whilst in men, and in women 
whose voices have a masculine roughness, the cartilages are 
harder, and are sometimes almost completely ossified. The loud¬ 
ness of the voice depends in part upon the force with which the 
air is expelled from the lungs; but the variations in strength 
of voice which exist among different individuals, are in some 
measure due to the degree in which its resonance is increased 
by the vibration of the other parts of the larynx and of the 
neighbouring cavities. In the Howling Monkeys of America, 
there are several pouches which open from the larynx, and are 
destined to increase the volume of tone that issues from it; 
one of these is excavated in the substance of the hyoid bone 
itself, which is very greatly enlarged; and to this bony drum 
seems due the mournful plaintiveness which characterises the 
