104 



In chronic roaring the noise is made wneu the air is drawn .nto the 

 lungs; and only when the disease is far advanced is a sound produced 

 when the air is expelled, and even then it is not near so loud as during 

 inspiration. 



In a normal condition the muscles dilate the aperture of the larynx 

 by moving outward the cartilage and vocal cord, allowing a sufficient 

 volume of air to rush through. But when the muscles are paralyzed 

 the cartilage and vocal cord that are nominally controlled by the 

 affected muscles remain stationary; therefore when the air rushes in 

 it meets this obstruction, and the noise is produced. When the air is 

 expelled from the lungs its very force pushes the cartilage and vocal 

 cords out, and consequently noise is not always produced in the expi- 

 ratory act. 



The paralysis of the muscles is due to derangement of the nerve that 

 supplies them with energy. The muscles of both sides are not sup- 

 plied by the same nerve; there is a right and a left nerve, each supply- 

 ing its respective side. The reason why the muscles on the left side 

 are the ones usually paralyzed is owing to the difference in the ana- 

 tomical arrangement of the nerves. The left nerve is much longer 

 and more exposed to interference than the right nerve. This pair of 

 nerves is given off from its parent trunks (the pneumogastrics) after 

 the latter enter the chest, and consequently they are called the recur- 

 rent larjmgeal nerves, on account of having to recur to the larynx. 



In chronic roaring there is no evidence of any disease of the larynx, 

 other than the wasted condition of the muscles in question. The 

 disease of the nerve is generally located far from the larynx. Disease 

 of parts contiguous to the nerve along any part of its course vasiy 

 interfere with its proper function. It is not really necessary for the 

 nerve itself to be the seat of disease; pressure upon it is sufficient; 

 the pressure of a tumor on the nerve is a common cause. Disease of 

 lymphatic glands within the chest through which the nerve passes on 

 its way back to the larynx is the most frequent interruption of nerv- 

 ous supply, and consequently roaring. When roaring becomes con- 

 firmed medical treatment is entirely useless, as it is impossible to 

 restore the wasted muscle and at the same time remove the cause of 

 the interruption of the nervous supply. Before the disease becomes 

 permanent it may be benefited by a course of iodide of potassium, 

 when it is caused by disease of the lymphatic glands. Electricity has 

 been used with indifferent success. Blistering or firing over the 

 larynx is of course not worthy of trial if the disease is due to inter- 

 ference with the nerve supply. The administration of strychnia (nux 

 vomica) on the ground that it is a nerve tonic, with the view of stimu- 

 lating the affected muscles, is treating only the result of the disease 

 without considering the cause, and is therefore useless. The opera- 

 tion of extirpating the collapsed cartilage and vocal cord is believed 

 to be the only relief, and as this operation is critical and can only be 

 performed by the skillful veterinarian, it will not be described here. 



