340 



THE SENSES 



the vertebral column are very numerous and in all directions. It is 

 enough to know that for the most part the muscles producing these 

 movements are small and therefore perfectly adapted for any desired 

 movement. They are controlled by what is practically reflex action. 

 The afferent impulses come especially from the retinae, but also from the 

 voluntary cortex and from many other places under the influences of 

 many different sorts of stimuli, for example from the ears. 



EYE-MCTVEMENTS have been the subject of a large amount of careful 

 and ingenious study, especially by the psychologists. For example, 

 Delabarre attached to his cornea a minute concave mirror with a hole in 

 its center and studied the directions taken by a ray of light reflected from 

 it on to a photographic plate. The movements, however, occur so often, 

 so quickly, so unconsciously, and are often of such small extent, that real 

 progress in the subject has been slow. Knowledge of these movements 

 is of great importance in many theoretical and practical problems, for 

 example the ideal mode of composing printed matter for being read. 



FIG. 194 



Diagram of the actions of the external muscles of the eye-ball. The figures in parentheses 

 indicate the nerves which supply the muscles. (Waller.) 



Fig. 194 shows well the gross movements as produced by the six 

 extrinsic eye-muscles. The nearly spherical globe of the eye is set in a 

 soft bed of fat and of vascular tissue and rotates with great readiness 

 under the influence of the muscles (the superior, inferior, internal, and 

 external recti, and the superior and inferior obliques) attached to it. 

 Thus, the internal and external recti rotate the eye in the directions their 

 names indicate; the superior rectus rotates it upward and inward, and 

 the inferior rectus downward and inward ; the superior oblique rotates it 

 downward and outward, and the inferior oblique upward and outward. 

 In the majority of cases, if not in all, the muscles work in combination, 

 and are always in such a state of balancing tonus as to hold the eye 

 functionally, but not actually, still. The muscles of the two eyes always 

 move together, but their contractions become less effective in old age. 

 All movements normally take place about the rotation-point, 11 mm. 



