718 SAtfDSOOK 01? PHYSIOLOGY. 



sympathetic, or of its centre in the floor of the front of the aqueduct of 

 Sylvius. The contraction of the pupil appears to be under the control 

 of a centre in the bulb or in the corpora quadrigemina, and this is 

 reflexly stimulated by a bright light, and the dilatation when the reflex 

 centre is not in action is due to the more powerful sympathetic action ; 

 but in addition, it appears that both contraction and dilatation may be 



Fig. 427. Diagram of the axes of rotation to the eye. The thin lines indicate axes of rotation, 

 the thick the position of muscular attachment. 



produced by a local mechanism, upon which certain drugs can act, which 

 is independent of and probably often antagonistic to the action of the 

 central apparatus of the third and sympathetic nerve. The action of the 

 fifth nerve upon the pupil is not well understood, but its apparent effect 

 in producing dilatation is due to the mixture of sympathetic fibres 

 with its nasal branch. The sympathetic influence upon the radiating 

 fibres is believed to be conveyed not by the long ciliary branches of that 

 nerve, but by the short ciliary branches from the ophthalmic ganglion. 

 The close sympathy subsisting between the two eyes is nowhere better 

 shown than by the condition of the pupil. If one eye be shaded by the 

 hand its pupil will of course dilate ; but the pupil of the other eye will 

 also dilate, though it is unshaded. 



DEFECTS IN THE OPTICAL APPARATUS. 



Defects in the Refracting Media. Under this head we may con- 

 sider the defects known as (1) Myopia, (2) Hypermetropia, (3) Astig- 

 matism, (4) Spherical Aberration, (5) Chromatic Aberration. 



