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 
u 2 
