518 OF RESPIRATION. 



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 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 prevent 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 office 

 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 applied to it produce expiratory movements; and this evidently operates 

 through the branches of the pneumogastric 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. 

 554. 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 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 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 contraction 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 some- 

 times 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 consequence of a series of short convul- 

 sive 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 movement, 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 arched, and the chest is more completely emptied 

 of air, than it could be by an ordinary movement of expiration. The act of 

 Crying, though occasioned by a contrary emotion, is, so far as the respiration 

 is concerned, very nearly the same as the last. Every one knows the effect of 

 mixed emotions, in producing an expression of them which is " between a laugh 

 and a cry." The greater part of the preceding movements seem to belong as 

 much to the consensual or to the emotional, as to the purely reflex group of 

 actions ; for whilst they are sometimes the result of peculiar states of the respi- 

 ratory organs, or of the bodily system in general, they may also be called forth 

 by influences which operate directly through the senses, or which excite the 

 emotions. Thus, whilst Sighing and Yawning often occur as simple' results of 

 deficient aeration, they may be brought on the former by a depressed state of 

 the feelings the latter by the mere sight of the act in another person. The 

 actions of Laughter and Crying seem never to originate in the respiratory system; 



