THE REGULATION OF BREATHING 295 



stimulation. Thus, the stimulation of the upper part of the larynx 

 by a crumb " going the wrong way " induces through the superior 

 laryngeal nerve inhibition of inspiration, followed by a fit of coughing, 

 by which the crumb is expelled. During swallowing, respiration is 

 inhibited; it is impossible to breathe and swallow at the same time. 

 This is owing to a reflex excited through the glosso-pharyngeal nerve. 

 vStimulation of the mucous membrane of the trachea and bronchi 

 also induces coughing, excited reflexly through the vagus nerve. 

 Stimulation of the mucous membrane of the nose with a mechanical 

 irritant induces sneezing, while a chemical irritant, such as an irre- 

 spirable gas e.g., a high percentage of C0 2 , ammonia vapour,, 

 chlorine, sulphur dioxide excites spasm of the glottis. 



Stimulation of the walls of the external auditory meatus* with a 

 foreign body or by a plug of wax may induce coughing a reflex excited 

 through a twig of the vagus nerve (the alderman's or Arnold's nerve). 

 A powerful light may excite sneezing. The " stomach cough '' is due 

 to reflex irritation of the vagus nerve supply to the stomach. " Hic- 

 cough," caused by a spasmodic contraction of the diaphragm, is 

 probably due to reflex stimulation of the centre, excited', perhaps, 

 by overdistension of the stomach. Persistent hiccough may occur in 

 case of severe illness e.g., carcinoma of the stomach, large haemorrhage 

 from the intestines, etc. 



The " winding " following a blow in the pit of the stomach has 

 been attributed to stimulation of the respiratory centre through the 

 splanchnic nerves. The "knock-out blow" on the chin is said to 

 jar the medulla oblongata. A powerful" electrical current passed 

 through the head may temporarily arrest the respiration, while, if it 

 pass through the heart, it may throw this into fibrillar contraction, 

 and so produce death. 



The nerve-supply of the larynx comes from the superior and from 

 the inferior (recurrent) laryngeal branches of the vagus. The superior 

 laryngeal is the sensory nerve to its mucous membrane, and furnishes 

 the effector supply to the crico-thyroid muscle. The recurrent laryn- 

 geal branch of the vagus supplies the motor fibres to the other muscles. 

 If this nerve be compressed or cut on one side, the voice is lost, because 

 the corresponding vocal cord cannot be adducted. Breathing is also 

 somewhat laboured, because it cannot be abducted, so that, when 

 laryngoscopic examination is made, the cord does not move as it 

 should to the middle line on phonation, nor away from it on inspira- 

 tion. When the nerve is gradually affected, the abductor muscle, 

 the posterior crico-arytenoid, fails first; in deep ether anaesthesia 

 the. adductor, the lateral crico-arytenoid, is most affected. Paralysis 

 of the superior laryngeal nerve, besides leading to loss of sensation, 

 causes hoarseness, due to deficient tension" of the vocal cords as the 

 result of paralysis of a crico-thyroid muscle. 



Any sudden and forcible stimulation of the skin modifies the 

 respiratory act e.g., the first plunge into a cold bath. It is customary 

 to flick with wet towels an infant which does not breathe after birth, 

 or to smack it. 



