g 
_-—-—=s 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, Ist, 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 
u2 
