DISEASES OF THE RESPIEATOBY SYSTEM. 157 



An animal driven for some distance at the top of his speed 

 (he being a willing horse) will overdo himself, and the residual 

 air has a tendency to dilate the air-cells. 



If drawing a heavy load, more muscular work must be per- 

 formed ; this causes a deep inspiration and a closure of the glottis, 

 and the air becomes heated and dilates and stretches the walls of 

 the air-cells. 



Coughing may cause it also, but it takes some time to produce 

 it in this way. 



The feeding of dusty or dirty hay or bulky food is also said 

 to produce it ; in the country they often feed dry hay, straw, corn- 

 stalks during the winter, and in the spring the horse may be heavy. 



It may follow pneumonia, pleurisy, etc. 



What are the post-mortem appearances? 



On opening the thorax, a striking difference in the appearance 

 of the lungs is seen ; instead of collapsing, as they do normally, the 

 lungs remain full, and even, in severe cases, may bulge up, as if they 

 were being inflated ; they are soft and downy to the feel, of a rosy 

 tint, but lighter than normal; they do not contain so much blood. 



On palpation they have a good deal of resistance, and pit some- 

 what on pressure; they are very light, and if a piece be cut off it 

 will float on water; it crepitates when cut. In the anterior, middle, 

 and posterior borders, the air-cells are enlarged. 



If it is the interlobular variety of emphysema, elevations which 

 contain air can be seen over the surface of the lungs; these eleva- 

 tions are often white, and old practitioners thought they were 

 tubercles; if cut, they contain air, press on them, and the air will 

 run under the pleura. 



What are the symptoms? 



These are very interesting for us, especially in cases of exam- 

 ination for soundness, as such horses are to be considered unsound. 



The disease comes on slowly or suddenly. The derangement 

 occurs in the mechanism of the respiratory movements which can 

 often be detected by simple inspection of the abdominal walls in the 

 region of the flank. The alteration is principally in the expiratory 

 movements. Normally, there is an inspiration, an expiration, and 

 then a pause; in this disease, during the expiration (falling in of 

 the hypochondriac region, hollowing of the upper part of the 

 flank), there is a short pause and a secondary expiratory move- 



