MOVEMENTS OF EYELIDS. 223 



that as, when mentation is paralyzed, horizontal 

 movements are most common, the vertical ones are 

 more expressive of mental states. The horizontal 

 movements of the eyes do not involve movements 

 of the eyelids; these movements are less compli- 

 cated than those in the vertical plane, and involve 

 the action of fewer nerve-centres. 



As to the mechanism of vertical movements, 

 Dr. Gowers * has shown that the movements of the 

 lower eyelid constitute a simpler problem than 

 those of the upper lid. The lower lid follows the 

 movements of the eyeball upwards and downwards, 

 but not very closely. No muscular mechanism 

 exists which can cause the downward movements 

 of the lower lid; such movement is produced by 

 the movement of the eyeball acting mechanically 

 upon the lower lid. The upper eyelid possesses 

 a more complex mechanism. The descent of the 

 lower lid, or downward rotation of the globe, is not 

 due to the contraction of a muscle, but simul- 

 taneously with the descent of the upper lid in the 

 downward movement of the eyeball, there must be 

 a relaxation of the elevator. In upward rotation of 

 the eyeball, contraction of the elevator is associated 

 with that of the superior rectus. The association 

 of the elevator and superior rectus suggests that 

 both are relaxed, or energized, in similar degree 

 when the eyeball is moved upwards or downwards. 

 Thus, in the upward vertical movements of the 

 eyeballs, a more complex ^mechanism comes into 

 play than with horizontal movements. 

 * Med.-Chir. Trans., 1879. 



