148 A MANUAL OF VETERINARY PHYSIOLOGY 



larynx that the epiglottis should exist ; it has been removed both 

 by disease and experimentally, and its place is then taken by 

 the base of the tongue. Nor is an arytenoid cartilage essential 

 to safety in swallowing. 



The Nervous Mechanism of the Larynx is peculiar. Sensation 

 to the mucous lining membrane and motor power to the crico- 

 thyroid muscle is supplied in the majority of animals by the 

 superior laryngeal branch of the vagus, this nerve containing both 

 sensory and motor fibres. In the horse the motor fibres running 

 in the superior laryngeal are, it is said, derived from the first 

 cervical nerve and not from the vagus. All the other muscles, 

 both abductor and adductor, are supplied with motor power by 

 the inferior or recurrent laryngeal branch of the vagus. It is 

 strange that both abductor and adductor muscles should have 

 the same source of nerve supply, and one naturally asks what it 

 is which determines that only the opening or only the closing 

 muscles shall act at any given moment ? The explanation of 

 this fact lies in the law of ' reciprocal innervation/ demonstrated 

 for the limb muscles by Sherrington, which will be considered 

 later in the chapter on the Nervous System. Both dilator and 

 constrictor fibres run in the recurrent laryngeal nerve, and are 

 quite distinct ; in some animals the different bundles have been 

 experimentally isolated and injured- — injury to the dilator fibres 

 producing abductor paralysis, and injury to the fibres going to 

 the muscles which close the larynx, producing adductor paralysis. 



If the recurrent laryngeal be cut and the peripheral end 

 strongly stimulated, the glottis almost invariably is found to 

 close ; in other words, only the adductor fibres appear to be acted 

 upon. If a weak stimulation be applied, the glottis opens — viz., 

 the abductor muscles are affected. 



Another curious fact in the history of the recurrent nerves 

 is furnished by pathology. In the disease of horses known as 

 1 roaring,' there is paralysis of the left abductor muscle of the 

 larynx — viz., the crico-arytenoideus posticus, the wasting and 

 fatty degeneration due to paralysis being very marked. It is 

 not unusual to find the adductor muscles normal in appearance, 

 or presenting very little sign of disease, and even if pale and 

 wasted the degree of degeneration cannot be compared with 

 that furnished by the abductor muscle. This is a difficult fact 

 to explain ; one would think that as both abductor and adductor 

 muscles receive the same nerve supply, equal wasting would 

 occur in both groups. Again, it is observed when the recurrent 

 has been divided experimentally that the abductor muscle 

 loses its irritability long before the adductors, and the same fact 

 may be observed in post-mortem stimulation of the nerves. If 

 the recurrent laryngeal nerves be divided under ether, and the 



