REFLEX ACTIONS. RESPIRATORY MOVEMENTS. 297 



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 pro- 

 ducing 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 con- 

 sensual or emotional, as to the purely reflex group of actions ; for whilst they 

 are sometimes the result of peculiar states of the respiratory 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 never seem to originate in the respiratory system ; but 

 to be always either expressions of the emotions, or simple results of sensa- 

 tions, crying being connected with the sense of pain, and laughter with that 

 of tickling. The origin of the act of Hiccup does not seem very clear; but 

 the movement is probably of a purely reflex nature. 



381. The purposes of the acts of Coughing and Sneezing are, in both 

 instances, to expel substances from the air-passages, which are sources of irri- 

 tation there; and this is accomplished in both, by a violent expiratory effort, 

 which sends forth a blast of air from the lungs. Coughing occurs, when the 

 source of irritation is situated at the back of the mouth, in the trachea, or 

 bronchial tubes. The irritation may be produced by acrid vapours, or by 

 liquids or solids, that have found their way into these passages ; or by secre- 

 tions which have been poured into them in unusual quantity, as the result of 

 disease; or by the simple entrance of air (especially if cold), when the mem- 

 brane is in a peculiarly irritable state. Any of these causes may produce an 

 impression upon the excitor fibres of the Par Vagum, which, being conveyed 

 to the Medulla Oblongata, shall give rise to the transmission of motor impulses 

 to the several muscles, that shall combine them in the act of coughing. This 

 act consists, 1st, in a long inspiration, which fills the lungs; 2d, in the 

 closure of the glottis at the moment when expiration commences ; and 3d, in 

 the bursting open (as it were) of the glottis, by the violence of the expiratory 

 movement; so that a sudden blast of air is forced yp the air-passages, carrying 

 before it anything that may offer an obstruction. The difference between 

 coughing and Sneezing consists in this, that in the latter, the communication 

 between the larynx and the mouth is partly or entirely closed, by the drawing 

 together of the sides of the velum palati over the back of the tongue; so that 

 the blast of air is directed, more or less completely, through the nose, in such 

 a way as to carry off any source of irritation that may be present there. It is 

 difficult to say how far these actions are simply reflex; or how far they -may 

 require the stimulus of sensation for their performance. 



382. Deglutition and Defecation. Another very important function of the 

 Spinal Cord (and of the ganglia corresponding to it in the Invertebrata), is the 

 control which it exercises over the entrance and termination of the Alimentary 

 Canal; and this reflex action might probably be traced in some animals, in 

 which the necessity for that of Respiration does not exist. In all beings which 

 are unequivocally of an animal character, a stomach or digestive cavity exists; 

 and a means must be provided for the introduction of food into it. This is 

 partly accomplished by the power, with which its entrance is endowed, of 

 contracting upon, and of attempting to draw inwards, whatever comes in con- 

 tact with it; as we may readily observe in the Star-Fish, or Sea-Anemone, 

 where what is commonly regarded as the mouth, is really the aperture of the 

 stomach. But we almost always find some more special apparatus, for bring- 



