THE RESPIRATORY SYSTEM. 407 
Yawning involves a deep-drawn, slow inspiration, followed 
by a more sudden expiration, with a well-known depression of 
the lower jaw and usually stretching movements. 
Sighing is much like the preceding, though the mouth is not 
opened widely if at all, nor do the stretching movements com- 
monly occur. 
Hiccough is produced by a sudden inspiratory effort, though 
fruitless, inasmuch as the glottis is suddenly closed. It is 
spoken of as spasm of the diaphragm, and when long continued 
is very exhaustive. 
Sneezing is the result of a powerful and sudden expiratory 
act following a deep inspiration, the mouth being usually closed 
by the anterior pillars of the fauces against the outgoing cur- 
rent of air, which then makes its exit through the nose, while 
the glottis is forcibly opened after sudden closure. It will be 
noticed that in most of these acts the glottis is momentarily 
closed, which is never the case in mammals during quiet res- 
piration. 
This temporary occlusion of the respiratory passages per- 
mits of a higher intrapulmonary pressure, which is very effect- 
ive in clearing the passages of excess of mucus, etc., when the 
glottis is suddenly opened. Though the acts described are all 
involuntary, they may most of them be imitated and thus 
studied deliberately by the student. It will also appear, con- 
sidering the many ways in which some if not all of them may ‘ 
be brought about, that if the medullary center is responsible 
for the initiation of them, it must be accessible by numberless 
paths. 
Comparative.—Few of the lower animals cough with the same 
facility as man, while laughing is all but unknown, crying and 
sobbing rare, though the whining of dogs is allied to the cry- 
ing of human beings. 
Sneezing seems to be voluntary in some animals, as squir- 
rels, when engaged in toilet operations, etc. , 
Barking is voluntary, and in mechanism resembles cough- 
ing, the vocal cords being, however, more definitely employed, 
as also in growling. 
Bawling, neighing, braying, etc., are made up of long expira- 
tory acts, preceded by one or more inspirations. The vocal 
cords are also rendered tense. 
