viii OCULAE MOVEMENTS 305 



(a) For rotation inward and upward, the internal rectus, 

 superior rectus, and inferior olili(|iu' ; 



(&) For rotation inward and downward, the internal rectus, 

 inferior rectus, and superior oblique ; 



(c) For rotation outward and upward, the external rectus, 

 inferior rectus, and inferior oblique ; 



(d) For rotation outward and downward, the external rectus, 

 inferior rectus, and superior oblique. 



These oblique movements may, as above stated, be associated 

 with a slight degree of rotation or rolling of the eye round the 

 horizontal visual axis. 



The recti muscles are stronger than the obliques, but this 

 difference is compensated by the fact that the axes of the latter are 

 more diagonal, and are therefore capable of rotating the eye more 

 vigorously round the visual axis. So that when the superior 

 rectus and inferior oblique, or the inferior rectus and superior 

 oblique, contract to the same strength, and act synergically, the 

 rotatory effect of the obliques is to a large extent eliminated. 



It must not be forgotten that this description of the action of 

 one, two, or three muscles in the movements of the eye is only a 

 simplification or diagrammatic representation of the facts, since it 

 assumes a common axis of rotation for each pair of muscles, 

 whereas in reality each muscle has its own axis. Eenieuibering 

 this we must admit with Volkmann that every movement of the 

 eye, even the simplest, requires the synergic and unequal contrac- 

 tion of a number of muscles. 



Moreover, the muscular actions which we have enumerated for 

 the production of single movements hold good only when the eye 

 is in the primary position at the outset, because the axis of rota- 

 tion of each muscle of course changes when it is in any other 

 position. Thus, for example, the contraction of the superior or 

 inferior oblique may suffice alone, with weak or negligible inter- 

 vention of the superior or inferior rectus, to rotate the eye up 

 and down when it is adducted ; when, on the contrary, the eye is 

 abducted, the sole or almost exclusive contraction of the superior 

 or inferior rectus suffices to rotate it upward or downward. 



Under normal conditions the movements of both eyes are 

 intimately associated ; they move simultaneously. Even when 

 one of them is blind, or when both have been excised, the muscles 

 of both contract in response to impulses of central origin. The 

 range of the movements diminishes somewhat with age. Mobility 

 is more restricted in the vertical than in the horizontal direction, 

 upward than downward. 



Under normal conditions, with the head upright, the move- 

 ments always take place so that the two visual axes lie in the 

 same plane. One eye cannot direct its visual axis higher or lower 

 than the other, nor is it possible for the two axes to diverge 



