224 DISEASES OF THE AIR-PASSAGES OF HORSES. 
mucous concretions and membraniform bands in the larynx, trachea, 
or bronchi; various productions, as coagula of blood, or concrete 
mucus or wounds, in any part of the canal; constriction from some 
tumour externally of the larynx, trachea, or bronchi, as by enlarge¬ 
ment of the thyroid or bronchial glands; osseous tumours growing 
upon the cervical vertebrae, See. And I have every reason to be¬ 
lieve that a diminished capacity of some part of the respiratory 
tubes exists very frequently, varying in degree, producing sensible 
alteration in the sound of the pulmonary expansion, and not suffi¬ 
ciently permanent to be obvious on examination after death, from 
contractions of a spastic nature of the fibrous tissues or muscles 
connected with them; and more especially when morbid secretions 
irritate the larynx or bronchi. 
Beside constriction of the air-vessels, we find dilatation of the 
bronchi. A bronchus may be dilated its whole length, or limited 
to a point: this recession of the air-vessel may vary from the size 
of a pea to that of a goose egg. I saw a very fine specimen of this 
kind in the lungs of a horse that were affected with chronic dis¬ 
ease : when I first observed the dilatation, I apprehended a suppu¬ 
rative excavation, as it was full of mucus of a puriform charac¬ 
ter ; but on washing it out, the parietes of the canal were sound, 
but much reduced in substance, whilst the walls in the other por¬ 
tions of the tube were evidently much thickened. Anterior and 
posterior to the dilated part, the tube was of its natural diameter. 
This animal had suffered from bronchitis some time previously, from 
the character of the mucous surface, if not the chronic form of it, 
up to the time he died. I am of opinion this alteration of the 
bronchi is only found in chronic disease of the lungs and air-tubes. 
The Causes of Congestion of the mucous membrane of the air- 
passages are not well known; nor does it often appear in a severe 
or general form, but more frequently in a partial state, and to usher 
in inflammatory action, laryngitis, cynanche trachealis, and bron¬ 
chitis. Veterinary surgeons are fond of giving to inflammation of 
the tissues of the air-passages such names as influenza, distemper, 
epidemic catarrh, and bewilder themselves about causes and influ¬ 
ences, the nature of which they can give no satisfactory explana¬ 
tion or proof of what they assert. For my own part, I prefer seek¬ 
ing causes from such sources as I can readily understand, and leave 
the study of Vesuvius irruptions and pestilential gases to those 
who are fond of inquiring into them. The same causes that give 
rise to catarrh produce laryngitis, as it is, for the most part, con¬ 
secutive of inflammation of the mucous surface of the nostrils. 
Among the predisposing causes are original conformation and dia¬ 
thesis ; disorder of the digestive and assimilating organs; cold, 
moist, changeable climates ; variable or wet seasons, especially 
