296 FUNCTIONS OF THE NERVOUS SYSTEM. 



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, irritation 

 of the mucous surface of the larynx, or of the trunks themselves, produced 

 contraction of the glottis and efforts to cough ; effects which were at once pre- 

 vented by dividing those nerves, and thereby cutting off their communication 

 with the Medulla Oblongata. There can be no doubt, then, that the superior 

 and inferior laryngeal branches constitute the circle of incidents 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 prevent the 

 entrance of improper substances; whilst the superior laryngeal nerve also ex- 

 cites 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 exajnple 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 excitability, so that stimuli ap- 

 plied to it produce expiratory movements; and this evidently operates 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. 



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

 sneezing, are nothing else than simple modifications of the ordinary movements 

 of respiration, excited either by mental emotions, or by some stimulus originat- 

 ing 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 moderate degree ; and 

 we notice it particularly, when the attention is released, after having been 

 fixed upon an object, which has excited it strongly, and which has prevented 

 our feeling the insufficiency of the ordinary movements 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 con- 

 traction of the muscles of the jaw, and also by a very great elevation of the 

 ribs, in which the scapulae partake. The purely involuntary character of this 

 movement is sometimes seen, in a remarkable manner, in cases of palsy ; in 

 which the patient cannot raise his shoulder by an effort of the will, but does 

 so in the act of yawning. Nevertheless this act may be performed by the 

 will, though not completely; and it is one that is particularly excited by 

 an involuntary tendency to imitation; as every one must have experienced, 

 who has ever been in company with a set of yawners. Sobbing is the con- 

 sequence of a series of short convulsive contractions of the diaphragm ; and it 

 is usually accompanied by a closure of the glottis, so that no air really enters. 

 In Hiccup, the same convulsive respiratory movement occurs ; and the glottis 

 closes suddenly in the midst of it; the sound is occasioned by the impulse of 

 the column of air in motion, against the glottis. In Laughing, a precisely 

 reverse action takes place; the muscles'of expiration are in convulsive move- 

 ment, more or less violent, and send out the breath in a series of jerks, the 

 glottis being open. This sometimes goes on, until the diaphragm is more 



