Causes of Chronic Roarivg. 117 



sympathetic system of nerves. But outside the chest there 

 is no difference between the right and left nerves, and up 

 to and on the larynx they have the same course, relations, 

 and distribution. Therefore it is to the thoracic differences 

 in the nerves that we must look for the exemption of one 

 side of the larynx, and implication of the other. 



I have stated that chronic Roaring is a neuropathic 

 disease, its nervous origin being undeniable. If it were due 

 to any local inflammation or swelling in the region of the 

 larynx, we should doubtless find the right side as frequently 

 involved as the left, and the morbid changes presenting 

 other characters than those described. Absence of mobility 

 of the arytsenoid cartilages and vocal cords might doubtless 

 be due to paralysis of the dilator muscles, from infiltration 

 of their fibres and nerve-filaments ; but this would be 

 obvious on post-mortem inspection. 



In chronic Roaring there are no indications whatever of 

 such inflammatory action, and the fact that very many 

 cases occur without any inflammation being present in the 

 larynx or its neighbourhood, is sufficient to prove that we 

 must seek for the causes elsewhere, and ascribe them to 

 other influences. Attributing the wasting of the muscles 

 on the left side of the larynx to some imperfection in the 

 condition of the nerve distributed to them, and also accept- 

 ing as a fact the statement that such imperfection, even 

 when slight, is more apt to affect the dilator than the con- 

 strictor muscles, it now remains to inquire what influences 

 render the left recurrent nerve defective. Here, however, 

 we come upon less certain ground, and must extend our 

 inquiry over a wide area, if we are to arrive at a satisfactory 

 conclusion. 



The causes which will render a nerve incompetent to 

 perform its function in one part of the body will do so in 

 another, and the left recurrent nerve cannot prove an excep- 

 tion. Experimentally, it has been demonstrated that divi- 

 sion or ligation of the right recurrent nerve in its course 



