382 RESPIRATION [CH. XXVI. 



Special Respiratory Acts. 



Coughing. In the act of coughing there is first of all a deep in- 

 spiration, followed by an expiration ; but the latter, instead of being 

 easy and uninterrupted, as in normal breathing, is obstructed, the 

 glottis being momentarily closed by the approximation of the vocal 

 cords. The abdominal muscles, then strongly acting, push up the 

 viscera against the diaphragm, and thus make pressure on the air in 

 the lungs until its tension is sufficient to noisily open the vocal cords 

 which oppose its outward passage. In this way considerable force is 

 exercised, and mucus or any other matter that may need expulsion 

 from the air-passages is quickly and sharply expelled by the out- 

 streaming current of air. The act is a reflex one, the sensory surface 

 which is excited being the mucous membrane of the larynx, and the 

 superior laryngeal nerve is the afferent nerve; stimulation of other 

 parts of the respiratory mucous membrane will also produce cough, 

 and the point of bifurcation of the trachea is specially sensitive. 

 Other sensory surfaces may also act as the "signal surface" for a 

 cough. Thus, a cold draught on the skin, or tickling the external 

 auditory meatus, in some people will set up a cough. 



Sneezing. The same remarks that apply to coughing are almost 

 exactly applicable to the act of sneezing; but, in this instance, the 

 blast of air, on escaping from the lungs, is directed, by an instinctive 

 contraction of the pillars of the fauces and descent of the soft 

 palate, chiefly through the nose, and any offending matter is thence 

 expelled. 



The " signal surface " is usually the nasal mucous membrane, but 

 here, as in coughing, other causes (such as a bright light) will some- 

 times set the reflex going. 



Hiccough is an involuntary sudden contraction of the diaphragm, 

 causing an inspiration which is suddenly arrested by the closure of the 

 glottis, causing a characteristic sound. It arises from gastric irritation. 



Snoring is due to vibration of the soft palate. 



Soiling consists of a series of convulsive inspirations at the moment 

 of which the glottis is partially closed. 



Sighing and Yawning are emotional forms of inspiration, the latter 

 associated with stretching movements of jaws and limbs. They appear 

 to be efforts of nature to correct, by an extra deep inspiration, the 

 venosity of the blood due to inactivity produced by ennui or grief. 

 Their contagious character is due to sympathy. 



There are many other abnormalities of the respiratory mechanism 

 which will become familiar to the student of medicine during his 

 clinical studies. We may mention as examples : laryngismus stri- 

 dulus (the spasmodic croup of children) ; this is a nervous affection 

 due to increased reflex irritability of the laryngeal mechanism ; the 



