ORGANS OF RESPIRATION AND VOICE. 18] 
arteries, two from the carotid and two from the subclavian, are distributed 
to it. Its proper function is not known, although some consider it to belong 
to the secreting glands, while others with more probability suppose it to be 
in some way concerned in sanguification. It is very subject to enlargement 
by disease, constitutiug the affection called bronchocele or goitre. 
2. Tue THyMus Bopy is another organ of ambiguous character, and only 
in its highest state of development in the fcetus before birth. It occupies 
the greater portion of the anterior mediastinum, and after birth decreases, 
until by the time an individual arrives at puberty it will have almost entirely 
disappeared. Some authors suppose its function to consist in the prepara- 
tion of a fluid, like chyle, and fitted for foetal growth before birth, and before 
chyle can be formed from food. 
4. THE FUNCTION OF RESPIRATION. 
The process of breathing consists of two distinct operations: inspiration, 
by which the external atmospheric air is brought into contact with the 
blood and yields up oxygen, and expiration, in which the carbonic acid 
formed in the combustion of the oxygen is exhaled together with other 
gaseous matters. The chemistry of breathing is a consequence of these 
physical operations. 
Inspiration, or the drawing in of the breath, is produced by the expansion 
of the thorax, by which a partial vacuum being effected, the air rushes into 
the lungs and inflates them. A subsequent contraction of the parietes of 
the thorax will cause the expulsion of the air thus introduced. The glottis 
is connected with the pharynx, from which there are two ways for the air 
to escape or enter, one through the nostrils, the other through the mouth. 
It is more usual, however, when not otherwise influenced, to inspire through 
the former and expire through the latter. At the instant of inspiration, 
the thorax enlarges in all its dimensions; the diaphragm is depressed or 
drawn down, the ribs are raised and drawn outwards, this involving a 
protrusion of the sternum. In expiration all parts return to their former 
position. The muscles especially concerned in respiration are, the dia- 
phragm, the intercostals, the levatores costarum, the scaleni, the serrati, 
the sterno-cleido-mastoid, longissimus dorsi, and the subclavius. The abdo- 
minal muscles are chiefly employed in expiration, but are assisted by the 
quadratus lumborum and the triangularis sterni. There are, however, 
other muscles concerned in respiration, especially such as fix the head, 
neck, and back, for the better action of the first set. In children, the abdo- 
minal muscles are more especially called into requisition, as shown in the 
rise and fall of the abdomen in breathing. In adult males this rise and fall 
are seen in the lower part of the thorax, while in women it is the upper part 
of the chest that heaves. 
The entrance and escape of air are accompanied by a peculiar sound, 
distinctly audible by applying a stethoscope to the chest. The normal 
sound thus observed varies materially in diseases of the lungs and air 
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