Pathology and Course of Chronic Roaring. 107 



three-quarters of an hour after death, the laryngeal muscles 

 of people who had died from cholera, that no contractions 

 at all could be obtained of the dilators, while the thyro- 

 arytaenoids responded well. 



This difference in the physiological characteristics of the 

 constrictors and dilators of the larynx, is, in all likelihood, 

 related to the very important and constant activity of 

 function demanded from, the latter. 



For how long a period the dilator muscles of the larynx 

 of the horse may retain their vitality when their nerves no 

 longer stimulate them, we have no exact knowledge. Bassi 

 found commencing atrophy of the laryngeal muscles two 

 months after division of the recurrent nerves, in a horse. 

 Experiment has demonstrated that, after section of the 

 motor nerve of an ordinary striped muscle, there is in- 

 creased excitability for mechanical stimuli until about the 

 seventh week, when it gradually diminishes, until it is 

 altogether abolished towards the sixth to the seventh 

 month. Fatty degeneration begins in the second week after 

 section of the nerve, and goes on until there is complete 

 muscular atrophy. 



In young horses. Roaring usually appears more suddenly 

 than in old ones, and has a tendency to become more 

 rapidly acute in them, the dyspnoea being also more severe. 

 This is no doubt owing to the difference in the condition of 

 the laryngeal cartilages, as has been already mentioned when 

 treating of the physiology of the organ and the results of 

 division of the recurrent nerves. In what may be termed 

 really chronic cases, the affection does not become modified 

 as the horse advances in years, but, on the contrary, has a 

 tendency to become more developed, for reasons which will 

 ' be given in the following chapter. 



The condition of the horse, weather, food, and other 

 circumstances, influence the noise emitted in respiration, 

 no less than the dyspnoea. Horses so affected roar less when 

 in good working condition, than when '-'soft" and fat. 



