Roaring and whistling are defects of respiration, arising out of a diseased condition of some portion of the air-passages, whereby one or the other of these sounds is produced, according to the nature and degree of the obstruction. As usually met with, it is a chronic and incurable disease, resulting from paralysis of the dilator muscles of the larynx. Less frequently it arises from other causes of a temporary character.
Perhaps no equine affection has attracted more attention from veterinary authorities than this, and the opinion is universally held by them that in a large measure heredity is responsible for its wide prevalence. Most people, whether interested in horses or not, have had the subject forced upon their attention from time to time in connection with turf celebrities, and if the hereditary character of roaring had been more generally accepted in the early days of horse-racing there is no doubt that both our thoroughbred stock and their half-bred produce would have been less subject to the malady than they are now known to be. So many celebrated roarers have gone to the stud that persons best acquainted with the stud-book tell us it is difficult to find a thoroughbred horse whose progeny are absolutely free of roarers. Be this as it may, the race-horse .of to-day is so susceptible that the slightest cough in a favourite animal spreads dismay among owners and trainers, and a large section of the general public not unfrequently share in the alarm. Chronic roaring is generally referable to wasting of the dilator muscles of the larynx, following upon a cold, an attack of influenza, or strangles, or some affection of the chest, all of which appear to have the effect of causing paralysis of the 'nerve of supply to the parts affected; or it may, and does, come about while an animal is in the best of general health. Numerous dissections prove that the left side of the larynx is almost invariably diseased, and the theory is suggested that this nerve (the left recurrent branch of the pneumogastric) is rendered specially liable to derangement, in consequence of its having to wind round one of the larger vessels (aorta) emerging from the heart before ascending the neck. This conclusion may or may not be the correct one, but the fact remains that the right nerve, which does not take this course, is seldom or never affected. Microscopic examination of the nerve trunk has failed to elicit any information as to the intimate cause of the paralysis, as no change in the structure of the fibres is observable, and we are left to assume that whatever interruption there may have been in the nerve current during life it was not of this nature. In its chronic form roaring prevails in males to a much greater extent than in females, and more frequently in stallions than geldings. It is seldom or never seen in ponies under 14 hands, and the liability to the disease increases with the height of the animal and the length of neck.
Fig;. 207. - Larynx of a " Roarer".