68 THE SURGICAL ANATOMY OF THE HORSE 



The left nerve passes between the aorta and the left bronchus, in 

 which situation it is related to the bronchial lymphatic glands, and 

 receives communicating filaments from the cardiac nerve. The left 

 nerve then runs forwards along the inferior aspect of the trachea and 

 passes out of the chest between the first pair of ribs. 



During its course up the neck each nerve is closely related to the 

 corresponding common carotid artery, along the inferior aspect of which 

 vessel the nerve runs. The inferior laryngeal nerve supplies all the 

 intrinsic muscles of the larynx with the exception of the crico- 

 thyroid. 



Some Diseases of the Larynx 



Laryngismus Paralyticus. — This is an affection of the larynx, other- 

 wise known as hemiplegia, or more commonly " roaring." It is 

 characterised by marked dyspnoea, and by an extraordinarily audible 

 respiratory sound which is heard during the inspiratory movements, 

 and which becomes more appreciable as the rapidity of the respirations 

 is increased, as, for instance, after a sharp gallop. 



It is an affection of uncertain cause. Post-mortem examination has 

 revealed the fact that in advanced cases all the intrinsic muscles on the 

 left side of the larynx, with the solitary exception of the crico-thyroid, 

 are usually found to have undergone a considerable degree of atrophy. 

 That all the muscles so affected should be supplied by the left inferior 

 (recurrent) laryngeal nerve, whilst the crico-thyroid is not supplied by 

 that nerve, has led us to naturally conclude that "roaring" is most 

 probably due to some injury leading to paralysis of this particular nerve. 



Now the intrinsic muscles on the right side of the larynx are not 

 usually affected, and since the right nerve exactly resembles the left in 

 the course which it takes up the neck from the entrance to the chest to 

 its distribution in the larynx, and also in the relationship which it bears 

 to its surrounding structures throughout this part of its course, several 

 theories explanatory of the causation of "roaring" have been advanced. 



