152 Veterinary Medicine. 



violent exertions and in excited states of the circulation generally. 

 Vaerst and Sussdorf show that the nerve is habitually flattened 

 between the posterior aorta and trachea, the effect being worst 

 when the heart's action is excited. 



The relatively great prevalence of roaring in the thoroughbred 

 English race horse at home (5% in the army in 1888) and in the 

 descendants of this race in other countries, implies a special cause 

 connected with conformation or usage. The spare habit of the 

 body, the lack of fat, and the violent exertion demanded of the 

 animal have been adduced in explanation. The last named is 

 commonly the most potent. A race of horses with great energy 

 and extraordinary speed is of necessity characterized by a large 

 cardiac development, and with steady and exacting training this 

 is increased to meet the demand. The more exacting the train- 

 ing, if kept short of exhaustion, and if successful in attaining 

 the highest possibilities of the animal, the greater the increase of 

 the heart. The larger and more powerful the heart the greater 

 its impulse with each contraction and the greater the liability to 

 injury of the nerve : ist, by the concussion of the heart itself ; and 

 2d, by the compression of the nerve between the posterior aorta 

 on the one hand and the trachea or pulmonary artery on the 

 other. The same cause operates in cases in dogs used in draught 

 (Miiller). Cases of laryngeal paralysis in man in connection 

 with hypertrophy of the right heart, are recorded. 



Horses suffer more numerously then mares especially in breed- 

 ing districts where the mares are rested during pregnancy and 

 nursing. 



A moderate wasting of the arytenoid muscles does not neces- 

 sarily cause roaring, especially in animals that are not subjected 

 to trials of speed or other violent exertion. Considerable atro- 

 phy of these muscles is often found in the subjects of dissection 

 in which the infirmity had not been suspected during life. This 

 adds especial emphasis to the unwontedly violent contractions of 

 the heart as a cause, whether this depends on pericardial, pleu- 

 ritic or pulmonary inflammation on the one hand, or on extreme 

 hard work on the other. This also serves to explain the compar- 

 ative absence of the symptom from cattle and dogs, that are not 

 compelled to severe exertion, since on the supervention of dys- 

 pnoea, such animals instinctively slacken their pace and thus an- 



