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Dr. T. L. Llewellyn. 



[Dec. 19, 



As the result of working for long periods in the comparative darkness of 

 the pit, the cells of the retina probably lose their power of producing 

 sufficient pigment for exact vision. This failure occurs sooner in fair blue- 

 eyed people and in people who, owing to refractive error, are subject to 

 a greater eye-strain. The more frequent occurrence of nystagmus in winter, 

 the loss of visual acuity, the dread of light, are all points in favour of this theory. 

 Another is the oblique position in which the head is held in many cases. 

 This is probably due to an attempt made by the patient to bring a fresh part 

 of the retina into play, the central portion being worn out : " In the dark 

 and with an absolutely homogenous field before them the eyes are always 

 moving " (Maddox, 29). These conditions nearly obtain in a coal mine. 



This diminished power of fixation calls for efforts by the higher centres 

 for better vision. Excessive accommodation and resulting eye-strain follows. 

 Darkness itself is not enough to set up nystagmus. 



Horses do not show any signs of the disease even after 10 years' continual 

 underground life. I examined 46 horses without result. 



Lower animals have no central fixation (Maddox) and the horse has- 

 apparently two fields of vision and consequently no need for associated 

 vision. They are also free from eye-strain. 



The position assumed by the miner, especially when the head is flexed and 

 the eyes look up (Jeaffreson), also has some influence. 



The total result is that the impulses passing to the brain are not so exact 

 as they should be and that the centres governing the associated movements 

 of the eyes are correspondingly disturbed. The intimate connection between 

 these centres is lost and incoordinate movements of the eyeballs result. 



Snell, Dransart, and others lay great stress on the fact that nystagmus is 

 best seen and often only brought out when the eyes are elevated, and they 

 attribute this to weakness of the elevator muscles. It is not the muscles 

 which are affected but the movements of elevation. 



This failure is most marked in elevation of the eyes for two reasons : — 

 (1) The movements of elevation are the weakest and least often used of all 

 the associated movements of the eyes ; when the centres are put to a severe 

 strain they are the first to go, (2) If the movements of the eyes are 

 noticed in the vertical plane it will be seen that in all positions below the 

 horizontal there is an associated convergence, while in all positions above there 

 is a tendency to divergence. The eyes are in a position of maximum stability 

 when depressed and converged, and consequently nystagmus is not found in 

 this position except in severe cases. 



In many cases, and in most long-standing cases, the head is thrown back r 

 the eyelid droops, and the patient looks out at you from half-closed eyes. He- 



