212 SPECIAL PHYSIOLOGY. 



surface of each cord is made to approach the other and to undergo 

 vibrations; moreover, the cords vibrate more actively, the escape of 

 air is more rapid, the notes cannot be so long sustained, and, lastly, 

 the force of the air is weaker. 



In old age, the muscular and nervous power are diminished, the 

 structural elements of the larynx undergo degeneration, its cartilages 

 become ossified, and other changes take place, which impair the 

 strength of the voice, causing it to lose its tone, and to become weak, 

 unsteady, and tremulous. The pitch of the voice is modified by 

 changes in the temperature, and by the condition of humidity of the 

 atmosphere ; in cold, damp weather, it is frequently lowered by two 

 or three notes. The voice of singers sometimes becomes dissonant ; 

 the muscles of vocalization being fatigued, are no longer under the 

 control of the will, and their movements become unsteady ; more- 

 over, the state of constant tension, to which the vocal cords are so fre- 

 quently subject, from over-exercise, induces certain changes in them, 

 which interfere with their healthy action. 



In certain diseases, the lining membrane of the larynx becomes in- 

 flamed, ulceration ensues, and the voice is impaired or lost ; so also in 

 oedema of the loose areolar tissue beneath the mucous membrane of the 

 glottis, which may be produced by inflammation of this membrane, or 

 by any obstruction to the venous circulation through these parts, the 

 voice is destroyed. 



The glottis, besides being the organ of voice, acts as a safety-valve, 

 preventing the accidental intrusion of foreign bodies, whether solid, 

 fluid, or gaseous, into the windpipe. Its sensibility, and that of the 

 parts immediately above it, is exceedingly delicate, so that it is admi- 

 rably adapted for this purpose. The moment any solid, fluid, vapor- 

 ous, or gaseous noxious substance comes in contact with the upper 

 opening of the larynx, sudden and spasmodic closure of the glottis 

 immediately takes place ; sometimes coughing ensues, and thus the 

 body is expelled from the air-passages. The closure of the glottis is 

 due principally to spasm of the arytenoid muscles ; probably, however, 

 other muscles are also concerned in it. 



The muscles of the larynx are placed under voluntary, emotional, 

 ideational, sensori-motor, and excito-motor, or so-called reflex, control, 

 by means of the superior and inferior laryngeal nerves, branches of the 

 pneumogastric, and of the motor fibres given to the pneumogastric by 

 the spinal accessory nerves. The functions of the laryngeal branches 

 of these nerves have been determined by numerous experiments on 

 animals. The superior laryngeal nerve is the so-called afferent nerve 

 of the larynx, supplying fibres to the highly sensitive mucous mem- 

 brane lining the air-passages in this situation ; it contains, moreover, 

 a few motor fibres for the supply of the crico-thyroid muscle, and in 

 part also of the arytenoid. The inferior laryngeal nerve is the effer- 

 ent or motor nerve of all the other muscles. Division of the inferior 

 laryngeal nerve is immediately followed by paralysis of all the mus- 

 cles of the larynx, except the crico-thyroid; the sensibility of the mu- 

 cous membrane of the glottis, however, remains unimpaired; division 

 of the superior laryngeal nerve causes a total loss of sensibility of the 



