112 CLINICAL DIAGNOSTICS. 



in the respiratory apparatus. IMost sensitive in this particular 

 is the superior laryngeal nerve, which is the sensory nerve of 

 the larynx, and the first three rings of the trachea. The mucous 

 membrane of the trachea is less sensitive, except at the bifur- 

 cation of the bronchi. The bronchi are just as easily irritated 

 as the larynx ; but cough can not be excited from 

 the parenchyma of the lungs. It can. however, 

 arise from the pleura when this organ is in a state of irritation. 

 Peripheral irritation is transmitted to the cough-center in the 

 brain, which innervates the expiratory muscles and recurrent 

 nerve, inducing the reflex spasm called cough. 



In exceptional cases cough can emanate from ter- 

 minals of the vagus nerve lying outside of the respiratory appara- 

 tus, as, for instance, from the external auditor}- meatus [ear], 

 nose, or abdominal organs. According to Albrecht cough can 

 occur from abscess in the liver. These are, however, exceptional 

 cases. Cough from the stomach has never been ob- 

 served in the horse. There is a possibility that cough may have 

 its origin in the brain. These exceptions are worthy of note and 

 should be considered in those cases of cough tlie cause of which 

 cannot be found to lie in the respiratory apparatus. 



Cough occurs: 



1. If foreign bodies are inhaled: smoke, dust (dusty 

 food), acrid gases (ammonia, sulphurous acid, chlorine, etc.). 



2. If cold air is inhaled, especially if the respiratory- 

 tract is inflamed: catarrhs of the trachea and bronchi, pleuritis, 

 traumatic injuries to the pleura (traumatic gastro-diaphragm- 

 itis of the ox). 



3. If miicus, exudate or foreign bodies (food) and para- 

 sites are present in the air passages: Gastrus larvae in the 

 larynx, Syngamus trachealis in the wind pipe, Strongyli in the 

 bronchi. 



In no case can cough originate when the sensory ter- 

 minals of the vagus nerve are no longer 

 susceptible to irritation. In severe phlegmonous 

 diseases of the mucous membrane, cough is absent. The cough 

 center in the brain must also be in normal condition. It is 

 disturbed when great mental depression 



