380 DISEASES OF BEEATHING. 



for many of them are big horses. From this it may be inferred 

 that aneurism of the aorta is not the usual cause 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 exposed to compression. He thus 

 explains why, in chronic roaring, the alterations observed are 

 nearly always Nin the left muscle of the larynx. 



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

 consists in its gradual conversion into white fibrous tissue, the 

 hard inelastic substance seen in a tendon or ligament. 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. 



A case is recorded of roaring produced by a melanotic tumour 

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

 pressed on the right reciUTent nerve. An instance is mentioned 

 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. Pressure of the 

 collar in draught does not injuriously affect these nerves. 



This paralysis arises on rare occasions from poisoning, which 

 will be discussed later on (p. 385). 



As causes of paralytic roaring may be included all diseases, 

 such as strangles, which give rise to tuftiours that might set up 

 this paralysis. The great immunity from roaring which horses of 

 hot climates possess (p. 381), shows that the usual causes of this 

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

 disposing influences, except on very rare occasions. 



Improper 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 performance 

 of one of its functions, influences the movements of the gullet 

 and stomach, it is reasonable to infer that the larynx will partici- 

 pate more or less in the ill-health of these organs. It is probable 

 that the fact of the left recurrent nerve extending further 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 they receive is, however, quite 

 soft, because it has been bruised and broken up, during the 

 native process of thrashing. These animals (as remarked when 

 discussing the question of heredity) hardly ever become roarers. 



