ANATOMICAL DESCRIPTION. 215 



The eyeballs lie in their sockets, the orbits of the 

 skull, resting among the fatty tissue which supports 

 them. If that fat be diminished in quantity, the 

 eyeball sinks further into the orbit ; if the fat 

 becomes congested and swollen up, it protrudes the 

 eyeball somewhat. The movements of the eyeball 

 are effected by small muscles attached to the eye 

 and arising from the wall of the orbit ; these small 

 muscles external to the eyeball are supplied with 

 motor force by the third, fourth, and sixth pairs of 

 nerves of the brain. The iris, or coloured portion 

 of the eye, is a muscular curtain, with an aperture 

 in its centre called the pupil. The movements of 

 this muscular apparatus, the iris, are under the 

 control of the sympathetic and third cranial nerves. 

 Stimulation of the sympathetic is followed by 

 dilatation of the pupil; stimulation of the third 

 nerve, or motor oculi, is followed by contraction ; 

 conversely, palsy of the sympathetic leads to a 

 small pupil, and palsy of the third nerve to a large 

 pupil. 



We have seen, then, that a very brief description 

 shows how the size of the pupil may be increased 

 or diminished by nerve-currents acting upon the 

 muscular apparatus of the iris. Such changes are 

 among the principal means of expression in the 

 eyeball. The varying size of the pupil is a mode of 

 expression. 



(1) Light falling upon the eye enters the pupil 

 and falls upon the retina, or nervous impressionable 

 layer of the back of the eye; this sends on a current 

 to the corpora quadrigemina of the brain, and is 



