498 



PHYSIOLOGY 



Experimental stimulation of these nerves causes the converse ell'eets to paralysis, which 

 therefore do not require specific description. 



When strabismus due to the complete paralysis of one of the recti is present, there 

 is not as a rule any difficulty in ascertaining which muscle is affected. When however 

 one of the oblique muscles is paralysed or when the paralysis is only partial, there may 

 be some difficulty in diagnosing the exact condition. It is found however that there 

 is a simple method by which the affected muscle may be found. This depends on t he 

 principle that if the eyes are rotated in thatdirection which requires complete contract ion 

 of the affected muscle, the strabismus will be found to get worse, owing to (ho 

 failure of that muscle being made v more pronounced ; if on the other hand the eyes are 

 turned in the opposite direction, the injured muscle is relaxed and the strabismus 

 vanishes. The following example will show the way the method is used : ( a man com- 

 plains of double 'vision following an injury to the eyes ; by directing the gaze in different 

 directions it is found that the double vision increases in amount as the eyes are turned 

 to the right. The injured muscle must therefore be clearly a dextro-rotator, that is, 

 the external rectus of the right eye, or the internal rectus of the left. Fig. 248 will 



L.S.R. 



/?..*. 



L.I.R.. 



R..INF. R.. 



L.3.0. R.S.07 



DOWN 



FIG. 248. Showing the direction in which paralysis affecting the different eye 

 muscles produces diplopia and the relative positions occupied by the true 

 and false images (Hartridge). Black shows image of ri^ht eye. white shows 

 image of left eye. The false image is always the one placed furthest from the 

 centre. 



be found of assistance, because the arrows which show the directions in which the 

 pi.i MPT.'. !>>, point to the names of the muscles the injury of which will set up the 

 c(.tilitiou which is found to evist. Experience shows that the injured eye is al\\ays 

 that to which the moiv deviated image belongs, and this fact may be readily ascertained 

 by placing in front of the right eye a slip of coloured glass. If the coloured ima^e is 

 found to be the one that is the more deviated, then it is the right eye that is involved 

 in the injury, and therefore in the case that we have been considering, the right external 

 rectus is the injured muscle. Conversely if the uncoloured image is found to be the 

 more deviated, then the injured muscle was the left internal rectus. In fact it is found 

 in every case that the injured muscle is the one which would give by its contraction 

 that position to the more deviated image which it is actually found to occupy. 



