STRUCTURE OF THE EYE 291 



Opposite the pupil is the macula lutea, or yellow spot 

 which surrounds a depression known as the fovea centralis. 

 Here only cones are present, and each fibre of the optic 

 nerve is connected only with one cone. This area is 

 further distinguished by the fact that there are no optic 

 nerve-cells or nerve-fibres directly beneath it, and that it 

 is devoid of blood-vessels. At the periphery of the retina 

 rods predominate. The fibres of the optic nerve converge 

 upon a point (the blind spot) just internal to the yellow 

 spot, where they pierce the choroid and sclerotic and form 

 the trunk of the nerve. Here too there is a depression — • 

 the optic cup, from the bottom of which enter and leave 

 the central artery and vein. At the optic cup there are 

 neither rods nor cones. 



Movements of the eyeballs are effected by the six ocular 

 muscles. These are the superior, inferior, external and 

 internal recti, which draw the eyeball upwards, downwards, 

 outwards and inwards respectively; the superior oblique, 

 which rotates the eyeball so that the eye looks outwards 

 and shghtly downwards ; and the inferior oblique, by which 

 the pupil is directed outwards and upwards. The lower 

 motor nerve-centres for these muscles are situated in the 

 grey matter surrounding the Sylvian aqueduct. Move- 

 ments of the eye muscles can be induced by stimulation 

 of several of the higher centres — notably the frontal lobe 

 and angular gyrus of the cerebrum and the deep nuclei 

 of the cerebellum. The movements thus induced always 

 involve both eyes in such a manner that the axes of 

 the eyes are parallel (conjugate deviation). This is owing 

 to the intimate connection which exists between the mid- 

 brain centres. Like the muscles of the hmbs, the ocular 

 muscles show reciprocal innervation, contraction of one 

 muscle being associated with relaxation of its antagonist. 

 Contraction of the left external rectus is accom])anied by 

 contraction of the right internal rectus and inhibition of 

 the left internal and right external recti. 



Voluntary movements of normal eyes are always con- 



