142 PHYSIOLOGY CHAP. 



sufficiently close together, even when the arytenoid muscles are 

 paralysed. 



Stimulation of the peripheral branch of a recurrent nerve 

 brings the cord of the same side nearer the median line than does 

 simple section of this nerve, while stimulation of both recurrent 

 nerves causes the cords to come together and the glottis to close. 

 So that normally the effect of the recurrent nerves which contain 

 fibres for both the abductors and the adductors of the glottis is 

 dominantly on the dilatators ; when, on the other hand, they are 

 stimulated artificially the effect on the adductors of the glottis pre- 

 dominates. The explanation of these phenomena seems to be as 

 follows : Normally, only those fibres of the recurrent nerves which 

 are connected with a centre intimately related with the bulbar 

 respiratory centre exert a constant tonic influence which maintains 

 the inspiratory dilatation of the glottis ; when, on the contrary, the 

 two recurrent nerves are artificially excited, all the laryngeal 

 muscles concerned in voluntary phonation (except the anterior 

 crico-thyroids) contract, and the contraction of the adductors 

 consequently predominates. 



Section of the superior laryngeal nerve on one or both sides 

 does not appreciably affect the glottis, but it makes the voice 

 raucous and prevents the formation of high notes owing to the 

 loss of function of the crico- thyroid muscles which keep the cords 

 in tension. Longet demonstrated that the peculiar harshness 

 which ensues on paralysis of the superior laryngeal nerve depends 

 wholly on its external branch, which gives fibres to the crico- 

 thyroid. Isolated section of this nerve produces the same effect 

 as section of the whole nerve. He found, too, that the hoarseness 

 of the voice can be made to disappear by bringing the cricoid 

 artificially nearer the thyroid ; it is therefore obviously due solely 

 to relaxation of the vocal cords. After cutting the internal 

 branch of the inferior laryngeal, Longet could detect no appreci- 

 able change in the animal's voice, and electrical stimulation of 

 this branch produced no effect on the laryngeal muscles, though 

 Magendie held that the ramus intern us contains motor fibres for 

 the arytenoid muscle. 



The centres of the laryngeal fibres, both those which maintain 

 the laryngeal respiratory rhythm and those which control phona- 

 tion, lie in the bulb or medulla oblongata. 



The centre for respiratory rhythm is closely connected with 

 the respiratory centre, but is distinct and independent of it. We 

 saw that the glottis, during quiet respiration, is kept constantly 

 dilated by the tonic action of the recurrent nerves. Semon and 

 Horsley, experimenting on cats, further showed that stimulation 

 of the upper portion of the floor of the fourth ventricle produces 

 marked widening of the glottis, but the thoracic respiratory move- 

 ments continue ; the bulbar centre for the laryngeal respiratory 



