CHAP. II.] coughing: a cold, its progress. 103 



34. Farther towards its lower extremity, the 

 wind-pipe becomes more membranous, but less 

 sensible of injury, and the cartilaginous rings gra- 

 dually lose their form : they no longer describe a 

 circle, being composed almost wholly of strong 

 elastic membrane, that it may bend out of the way 

 when the gullet is distended with swallowing. Its 

 internal surface is lined with a membrane, which 

 incessantly secretes a quantity of the oily or mucous 

 fluid spoken of in sections 20 and 21, thereby de- 

 fending its coats from the action of the air in pass- 

 ing to and from the lungs. But this secretion being 

 sometimes exhausted by the very great exertions of 

 the animal, he then coughs so as to shake his entire 

 frame, as if to incite the membrane to make fresh 

 secretions of fluid for its defence ; or, in default 

 thereof, the cold air still rushing in at each inspira- 

 tion, he contracts a permanent cold, or catarrh, 

 which if suffered to continue, increases and runs 

 along the membrane to the lower branches of the 

 pipe, and ultimately communicates its baleful in- 

 fluence to one or both lobes of the lungs. If the 

 attack be trivial, small green spots are found on the 

 surface of the lungs, which would form ulcers, in- 

 creasing in size and number, according to the num- 

 ber of small tubes or cells affected. These tubes 

 lose their functions in consequence of the first 

 attack, the animal's wind becomes worse every time 

 he is hard pushed, and the cells burst into each 

 other, until, perhaps, one lobe, or half of his lungs 

 is rendered useless. In process of time, it turns as 



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