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CHAPTER IV. 



DISEASES OF THE RESPIRATORY ORGANS. 



BY the expression "respiratory organs" is meant, not only the lungs themselves, but the 

 trachea or windpipe, the larynx or organ of voice, and the nares or nostrils. The trachea or 

 windpipe descends downwards and backwards to the interior of the chest, where it divides 

 into two large bronchi, one for each lung. The trachea is formed of a series of cartilaginous 

 rings joined together by ligaments, and lined with mucous membrane. The bronchi are also 

 lined with mucous membrane, and the lungs throughout their whole course. It is this mem- 

 brane which is the seat of inflammation in severe catarrh or bronchitis. 



No sooner have the bronchi entered the substance of the lungs than they commence 

 giving off innumerable branches, forming smaller and smaller tubes, getting less and less until 

 they end in what are termed the intercellular passages. If one looks at a leafless oak-tree it 

 will give a very good idea of the division and subdivision of the bronchial tubes. The stem or 

 trunk of the tree shall represent the windpipe, the larger branches the larger bronchi, the 

 smaller the smaller, and the ultimate ramifications, or twigs of the branches, the intercellular 

 passages, which are probably not more than one-fiftieth of an inch in diameter. 



Now there are, opening into these inter-cellular passages, innumerable little four-sided 

 cavities or depressions. These are called the alveoli, or air-cells. They are not larger than 

 between one two-hundredth and one-seventieth of an inch ; and it is all around these that 

 the minute capillary or hair-like blood-vessels are spread. And not only the bronchi large and 

 small, but the air-cells even, are lined with mucous membrane ; this, being a secreting membrane, 

 in health is always moist, the moisture or secretion enabling the membrane to throw off foreign 

 matters from its surface, such as fine dust. When the vessels that supply this membrane 

 become reddened and congested, the secretion is largely increased, as happens in bronchitis. 



Spasmodic contraction of these air-cells constitutes asthma, while dilation of them constitutes 

 the disease called emphysema or " broken wind." 



The theory of respiration is very simple, and may be described in few words. The venous 

 blood that is, the vitiated blood returned from the various organs and tissues of the body 

 is pumped by the heart into the lungs, where, through the medium of myriads of capillaries, 

 it is spread out around the air-cells to be revivified by the air which is breathed. This revivi- 

 fication is simply a chemical act. The oxygen of the air combines with the carbon (or car- 

 bonaceous matter or impurities) of the venous blood to form carbonic acid (CO 3 ), which is ex- 

 haled. The carbon is burned off, so to speak, and the blood is not only purified, but heated, and 

 thus returned to the heart, to be re-distributed to every portion of the body. Respiration thus 

 becomes not only the purifier of the blood, without which purification it would speedily produce 

 death, but also a source of heat. Formerly it used to be considered the only source of heat in the 

 body ; now it is pretty well known that there is a union of oxygen with carbon and hydrogen, 

 which takes place in the tissues themselves and evolves heat, and there are several other sources 

 of heat which need not here be enumerated. 



