460 MUSCULAR MOTION. 



m 



it, otherwise we should have the vocal sound accompanying each con- 

 traction of the chest. Volition is necessary to excite the requisite action 

 of the muscles of the larynx, as well as those of respiration ; and by it 

 the tone and intensity of voice are variously modified. 



That voice is produced in the larynx, we have both direct and indirect 

 testimony. An aperture made in the trachea, beneath the larynx, 

 deprives both man and animals of it. This occurs also, if the aperture 

 be made in the larynx beneath the inferior ligaments ; but if above the 

 glottis, so as to implicate the epiglottis and its muscles, the superior 

 ligaments of the glottis, and even the upper portions of the arytenoid 

 cartilages, voice continues. MM. Magendie 1 and J. Cloquet refer to the 

 cases of two men, who had fistulas in the trachea ; and who were unable 

 to speak unless the openings were accurately stopped by mechanical 

 means. If, again, we take the trachea and larynx of an animal or 

 man, and blow air forcibly into the tracheal extremity towards the 

 larynx, no sound is produced, except what results from the friction of 

 the air against the sides. But if we approximate the arytenoid carti- 

 lages, so that they touch at their inner surfaces, a sound is elicited, 

 bearing some resemblance to the voice of the animal to which the 

 larynx belongs; 2 the sound being acute or grave according as the car- 

 tilages are pressed against each other with more or less force; and 

 varying in intensity, according to the degree of force with which the 

 air is sent through the tube. In this experiment, the inferior ligaments 

 are seen to vibrate. 



Paralysis of the intrinsic muscles of the larynx likewise produces 

 dumbness; and this can be effected artificially. Much discussion at one 

 time prevailed regarding the effect of tying or cutting the nerves 

 distributed to these muscles. The experiments of Haighton 3 induced 

 him to think, that the recurrent branches of the par vagum supply parts, 

 which are essentially necessary to the formation of the voice; whilst 

 the laryngeal seemed to him to affect only its modulation or tone. 

 Subsequent experiments have sufficiently shown, that if both the recur- 

 rent nerves and the superior laryngeal are divided, complete aphonia 

 results. M. Magendie 4 found, indeed, that when both recurrents, which, 

 he says, are distributed to the thyro-arytenoid muscles, are cut, the 

 voice is usually lost; whilst if one only be divided, the voice is but half 

 destroyed. He noticed, however, that several animals, in which the 

 recurrents had been cut, were still capable of eliciting acute sounds, 

 when labouring under violent pain,; sounds, which were analogous to 

 those that could be produced mechanically on the larynx of the dead 

 animal, by blowing into the trachea and approximating the arytenoid 

 cartilages; and this he attempts to explain by the distribution of the 

 nerves to the larynx. The recurrents being divided, the thyro-arytenoid 

 muscles are no longer capable of contracting, and aphonia results; but 

 the arytenoid muscle, which receives its nerves from the superior laryn- 

 geal, still contracts; and, during a strong expiration, brings the ary- 



' Precis, &c., i. 241, and his Journal de Physiologic, ix. 119. 



5 Biot, Traite Elementaire de Physique, i. 462. 



3 Memoirs of the Medical Society of London, iii. 435. 4 Precis, &c., i. 243. 



