ORGANS OF RESPIRATION AND VOICE. 181 



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, constituting the affection called hronchocele or goitre. 



2. The Thymus Body 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 Eespiration. 



The process of breathing consists of two distinct operations : inspiratmi, 

 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 tht^ 

 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 



887 



