382 DISEASES OF BEEATHINGr. 



is not the usual oause of roaring in horses. " According to 

 Goubaux, the left recurrent is situated more superficially than the 

 right in the lower part of the neck, and for this reason is more ex- 

 posed to compression. He thus explains why, in chronic roaring, 

 the alterations observed are nearly always in the left muscle of the 

 larynx " {Chauveau). 



The wasting (atrophy) of a muscle from loss of nervous supply 

 consists in its gradual conversion into white fibrous tissue, which 

 is the hard inelastic substance we see in a tendon or liga- 

 ment. As fibrous tissue can never become reconverted into 

 muscle, a cure for the paralytic form of roaring is possible only 

 before this structural change has taken place, namely, in the first 

 stages of the complaint. 



Cadeao records a case of roaring produced by a melanotic tumour 

 (p. 128), which was on the upper third of the right side of the neck, 

 and which pressed on the right recxirrent nerve. Glechner men- 

 tions an instance of roaring due to pressure by a thrombus (p. 118) 

 in the jugular vein. An injury to a recurrent laryngeal nerve from 

 external violence has been known to produce roaring. Oadeac 

 points out that pressure of the collar in draught cannot injuriously 

 affect these nerves. 



This paralysis arises on rare occasions from poisoning, which 

 will be discussed later on (p. 387). 



As causes of paralytic roaring, we may include all diseases, such 

 as strangles, which are liable to give rise to tumours that might set 

 up this paralysis. The great immunity from roaring which horses 

 of hot climates possess (p. 383), shows that the usual causes of this 

 disease are unable to act without the aid of one or more predis- 

 posing influences, except on very rare occasions. 



Imjjroper food, generally of a dry and hard nature, appears to be 

 a cause of roaring. As the nerve affected in roaring is a branch 

 of the great nerve (the pneumogastric) which, in the per- 

 formance of one of its functions, influences the movements of 

 the gullet and stomach, it is reasonable to infer that the larynx 

 will participate more or less in the ill-health of these organs. 

 It is probable that the fact of the left recurrent nerve extend- 

 ing farther back into the chest than its fellow, is one of the 

 causes of its being more readily affected by indigestion than 

 the right recurrent. In Syria, Egypt, and other parts of the 

 East, horses are fed for about ten months of the year on barley 

 straw or wheaten straw, and a little barley; and during that time 

 they do not as a rule get any green food. The straw which they 

 receive is, however, quite soft, because it has been bruised and 

 broken up, during tlie native process of thrashing. These animals 

 (as I have remarked when discussing the question of heredity) 



