Causes of Chronic Roaring. 121 



damaging the nerve at the aorta is supposed to be further 

 supported by the observation of Sussdorf, that the left 

 recurrent nerve shows a flattening, perceptible by means of- 

 the microscope, where it passes between the aorta and 

 trachea. 



But the objection I have offered above applies also here. 

 English horses, however much they may be liable to become 

 affected in their own climate, appear to lose this predisposi- 

 tion in warm dry climates, and yet we cannot believe that 

 the anatomical relations of arteries and nerves are altered 

 with the change of latitude. Arab horses in training (as 

 they often are in India) cannot differ much, anatomically, 

 from English horses ; and yet Roaring is all but unknown 

 among them. Besides, nerves are not inelastic cords, but 

 can be extended to a considerable extent without their 

 function being interfered with. In small animals, the 

 recurrent nerve may be raised as much as half an inch, 

 without any effect being produced on the laryngeal 

 muscles. 



Notwithstanding these objections, however, there is 

 reason to suspect that there is some foundation for the 

 supposition in the case of very young thoroughbred race- 

 horses. The early age at which these animals — immature 

 as they are— begin to be treated as if fully mature ; their 

 stable management, feeding, training, and altogether very 

 unnatural existence, must severely test their organization ; 

 and it can scarcely be wondered at if the left recurrent 

 nerve, like all other organs and tissues, sometimes, or even 

 frequently, suffers overstraining and pressure, particularly 

 at the point where it lies in contact with the posterior 

 aorta. Fast racing at two and three years of age must 

 impose a very heavy task upon the heart and its large 

 vessels, at a time when they are only partially developed, 

 and their dilatation must be all the greater as their tissues 

 are soft and yielding. The posterior aorta, from the large 

 amount of blood it has to carry, must participate in the 



