Etiology. Pathogenesis, 731 



nerve passes roiiml the coisto-cervical artery. The nerve on the left side is covered 

 by the pleura almost up to its point of exit from the chesty and in the neck it is 

 placed much more superficially than the right nerve. There appear to be grounds 

 for the supposition that the longer nerve is more exposed to unfavorable influences 

 than the right. According to Martin the conductivity of the nerve may be disturbed 

 by pulsation of the aorta if that is excessive, or if the nerve is in close contact with 

 a dilated arch. According to Ellenberger the fat around the aorta disapj)oars when 

 a horse is in training and consequently the recurrent grasps the aortic arch more 

 closely. Some authors state that a long neck is a predisposing influence. They 

 hold that if the neck be very long the nerve is stretched and pressed more firmly 

 against the aorta. Finally, accortling to Martin, the backward displacement of the 

 heart during development of the other organs must be compensated for by the 

 nerves. 



Tliomassen's investigations furnish a more satisfactory 

 explanation of the facts observed. In the great majoritj- of 

 cases it must be supposed that the left recurrent nerve is far 

 more susceptible to toxins and infective materials. Experience 

 shows that in poisoning with riga pea or lead it is principally 

 or exclusively the left nerve that is affected and, further, in 

 the apparently primary cases of roaring there is inflammation 

 only in that portion of the nerve which is outside the cavity of 

 the chest, well away from the influence of the circulatory sys- 

 tem. The reason of the greater susceptibility of the left nerve 

 to toxins is not yet known. 



It appears to be very doubtful whether there is such a thing as roaring due 

 to a primary disease of the muscle, if one excepts the case (Glockner) in which 

 there was thrombosis of the carotid. The coincident disease of the muscles suspected 

 by Nocard is cjuite possibly the result of degeneration of the nerve set up by the 

 virus of strangles. On the other hand the cases of recovery that are considered by 

 some authors to furnish proof of the myopathic origin of the disease in reality do 

 not furnish any such proof since recovery of the nerves may take place. 



It is generally admitted that a tendency to the disease is hereditary, and in- 

 vestigations confirm this view. Cadeac and Nocard rightly point out that the fre- 

 quent occurrence of the disease in certain studs may be due to the frequent oc- 

 currence of strangles and pneumonia. Thus, in the Argentine Eepublic where roaring 

 occurs very rarely, imported stallions that are roarers are used for breeding with- 

 out the tendency to the disease being transmitted to their olfspring. Certain 

 breeds, especially well-bred animals, and English thoroughbreds appear to be more 

 liable to become affected than half-breds or others. Age appears to exercise some 

 influence on the occurrence of the disease, the majority of cases occurring in ani- 

 mals between the ages of 3 and 6 years. Old horses are affected with moderate 

 frequency, but it is very rare to find an animal affected during the second or 

 third year. Sex appears to have no influence on the occurrence of the disease. 



Miiller has seen the disease repeatedly in the dog. In one ease, 

 the animal had had an attack of broncho-pneumonia, due to distemper 

 five months previously; while another case was caused by dragging on 

 the collar while being led. Frick records a case due to a fall out of a 

 Avindow, and in a case recorded by Albrecht, the cause was not dis- 

 covered. Paralysis of the recurrent nerve has been observed in cattle 

 by Vitz, Ollmann, Besnoit and Prietsch, the cause being enlargement 

 of the peribronchial lymphatic glands. 



Pathogenesis. Since the recurrent nerve is a mixed nerve 

 the excitability of the sensory fibers is first increased, the in- 

 creased irritability being in proportion to the rapidity with 

 which the process develops. If the disease develops slowly, 

 there may be only very slight irritability of these fibers. Sooner 



