CHRONIC ALVEOLAR EMPHYSEMA 49 



causes the expiration to become forced. These two factors 

 in time lead to weakening and finally atrophy of the alveolar 

 walls. The alveoli can be ten times their normal diameter, 

 the walls very thin and anemic. Later rupture of the 

 interalveolar septa occurs permitting direct communication 

 between the distetided air cells. The lung thus loses its 

 power of contraction at expiration, which causes this act to 

 become forced so that the air from the emphysematous 

 part of the lung is expelled, (b) Severe dyspnea due to 

 diseases of the larynx or bronchi is a rare cause, (c) Severe 

 coughing, especially in chronic bronchitis, {d) Occasionally 

 chronic alveolar emphysema results from the acute form, 

 (e) The feeding of bulky forage, particularly clover or 

 dusty timothy hay is no doubt an important predisposing 

 cause of pulmonary emphysema, the dilatation of the 

 stomach and bowels which such foods produce interfering 

 with respirations. (/) As some strains of horses seem more 

 subject to emphysema than others, an inherited predisposi- 

 tion (lacking resistance in the septa) has been assumed. 

 Of this, however, there is no substantial proof. Use and 

 feeding methods may explain the assumption. 



Cases of "heaves" have occiured among horses which were 

 never exercised vigorously or overexerted. In fat, old, well 

 kept carrisige horses and pet ponies it is relatively more com- 

 mon than among highly bred race or hunting horses. Horses 

 fed bulky foods, especially hay made of tame grasses, seem 

 most subject to the disorder. It has been stated that before 

 tame grasses were introduced into the far Western States 

 "heaves" was unknown in that part of the country. While 

 "heaves" does occur occasionally among very yoUng horses, 

 it is essentially a disease of older age increasing in frequency 

 with age. A great many cases of "heaves" are combined 

 with chronic bronchitis. 



Symptoms. — Dyspnea which increases on exercise. It is 

 always most pronounced at expiration and is often accom- 

 panied by a double movement of the flanks and the inter- 

 rupted protrusion of the anus. The first part of the expira- 

 tory act is passive, but after a very brief pause, the movement 

 becomes active, the abdominal muscles contracting with 

 4 



Digitized by Microsoft® 



