194 OPHTHALMOLOGY FOR VETERINARIANS 



when the normal half of the retina is shaded, determines 

 the location of the pressure or disease. The pupil fails 

 to react to light when pressure is anterior to the so-called 

 * 'reflex arc/' that is, anterior to the origin of the third 

 nerve. 



The fields of vision are greatly contracted in glaucoma 

 and in diseases of the optic nerve and retina, but in a 

 work of this nature it is unnecessary to go into such 

 details. 



Scotoma. — A portion of the field of an eye may be 

 wiped out by reason of disease or atrophy of that portion 

 of the retina which should receive it. Such a condition 

 is known as a scotoma. For instance, a hemorrhage 

 may occur in the macula which will produce a central 

 scotoma; that is, the object in the direct visual axis 

 cannot be seen, while the other fields are preserved. If 

 the macula is normal, but disease and atrophy occur 

 in some other portion of the retina, the central field will 

 be preserved, but that portion of the field which is 

 received by the diseased portion of the retina will be 

 obhterated. 



