COUGHING AND SNEEZING AQUEOUS EXHALATION. 291 



disease ; and the latter will be the more likely to produce the 

 effect, from the irritable state in which the lining membrane 

 of the air-passages already is. The impression made upon 

 this membrane is conveyed by the nerves spread out beneath 

 its surface to the spinal cord; and the motor impulses are 

 sent to the different muscles, which they combine 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 burst- 

 ing-open, as it were, of the glottis, by the violence of the 

 expiratory movement, so that a sudden blast of air is forced 

 up the air-passages, carrying before it anything that may offer 

 an obstruction. Sneezing differs from coughing in this, that 

 the communication between the larynx and the mouth is 

 partly or entirely closed, by the drawing-together of the sides 

 of the veil of the palate 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. 



343. Every one is aware that the air he breathes-forth con- 

 tains a large quantity of vapour : this is not perceptible in a 

 warm atmosphere, because the watery particles remain dis- 

 solved in it and do not affect its transparency ; but in a cold 

 atmosphere they are no longer held in solution, and conse- 

 quently present the appearance of fog or steam. The quantity 

 of fluid which thus passes off is by no means trifling, 

 probably not less than from 16 to 20 ounces in the twenty- 

 four hours; a portion of it undoubtedly proceeds from the 

 moist lining of the mouth, throat, &c., but the greater part 

 is thrown-off by the lungs themselves. This fluid, when col- 

 lected, is found to contain a good deal of decomposing organic 

 matter, especially in cases in which the respiratory process 

 has not been carried on with perfect freedom; such matter 

 being oxydized and thrown-off under other forms, when the 

 blood is duly aerated. Various substances of an odoriferous 

 character, which have been taken into the blood, manifest 

 their presence in this exhalation : thus turpentine, camphor, 

 and alcohol, communicate their odour to the breath; and 

 when the digestive system is out of order, the breath fre- 

 quently acquires a disagreeable taint, from the reception of 

 putrescent matters into the blood, and their exhalation through 



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