RESPIRATORY MOVEMENTS. 143 



cles. After the first paroxysm occasioned by the operation, a period of qui- 

 escence and freedom from dyspnoea often supervenes, the respirations being 

 performed with ease so long as the animal remains at rest ; but an unusual 

 respiratory movement, such as takes place at the commencement of a struggle, 

 induces immediate symptoms of suffocation, the current of air carrying in- 

 wards the arytenoid cartilages, which are tendered passive by the paralyzed 

 state of their muscles ; and these falling upon the opening of the glottis like 

 valves, obstruct the entrance of air into the lungs. ' The more effort is made, 

 the greater will be the obstruction : and accordingly, it is generally necessary 

 to counteract the tendency to suffocation, when it is desired to prolong the 

 life of the animal after this operation, by making an opening into the trachea. 

 Dr. Reid further ascertained, that the application of a stimulus to the inferior 

 laryngeal nerves, when separated from the trunk, would occasion distinct 

 muscular contractions in the larynx, whilst a corresponding stimulus applied 

 to the superior laryngeal occasioned no muscular movement, except in the 

 crico-thyroid muscle. But when the superior laryngeals were entire, irrita- 

 tion of the mucous surface of the larynx, or of the trunks themselves, pro- 

 duced contraction of the glottis and efforts to cough ; effects which were at 

 once prevented by dividing those nerves, and thereby cutting off their com- 

 munication with the medulla oblongata. There can be no doubt, then, that 

 the superior and. inferior laryngeal branches constitute the circle of incident 

 and motor nerves, by which the aperture of the glottis is governed, and by 

 which any irritation of the larynx is made to close the passage, so as to pre- 

 vent the entrance of improper substances ; whilst the superior laryngeal nerve 

 also excites the muscles of expiration, so as to cause the violent ejection of a 

 blast of air, by which the offending gas, fluid or solid, may be carried off'. 

 The effect of carbonic acid in causing spasmodic closure of the glottis is well 

 known, and affords a beautiful example of the protective character of this 

 system of nerves. The mucous surface of the trachea and bronchi appears, 

 from the experiments of Valentin, to be endowed with impressibility, so- that 

 stimuli applied to it produce expiratory movements ; and this evidently ope- 

 rates through the branches of the par vagum distributed upon the membrane. 

 Here, as elsewhere, we find that a stimulus applied to the surface has a much 

 more decided influence than irritation of the trunk of the nerve supplying it. 

 Valentin has succeeded in producing distinct contractions of the ri$g$ of the 

 trachea, by irritating the par vagum in the rabbit ; and he thinks it "probable 

 that a similar action might be induced in the bronchi and their ramifications ; 

 but this he has not succeeded in procuring. The phenomena of asthma, how- 

 ever, leave little room for doubt, that spasmodic contraction of the air-passages 

 takes place as a reflex action, excited by various causes ; and no other nerve 

 but the par vagum can be concerned in producing it. 



189. The actions of sighing, yawning, sobbing, laughing, coughing and 

 sneezing, are nothing else than simple modifications of the ordinary move- 

 ments of respiration, excited either by mental emotions, or by some stimulus 

 originating in the respiratory organs themselves. Sighing is nothing more 

 than a very long-drawn inspiration, in which a larger quantity of air than 

 usual is made to enter the lungs. This is continually taking place to a mode- 

 rate degree ( 530) ; and we notice it particularly, when the attention is re- 

 leased, after having been fixed upon an object, which has excited it strongly, 

 and which has prevented our feeling the insufficiency of the ordinary move- 

 ments of respiration. Hence this action is only occasionally connected with 

 mental emotion. Yawning is a still deeper inspiration, which is accompanied 

 by a kind of spasmodic contraction of the muscles of the jaw, and also by a very 

 great elevation of the ribs, in which the scapulae partake. The purely in- 

 voluntary character of this movement is sometimes seen, in a remarkable man- 



