862 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



eye is not sufficiently great to bring parallel rays of light to a focus on 

 the retina. When such an eye is at rest only convergent rays are capable 

 of forming a distinct image on the retina, so that, therefore, distinct 

 images can only be formed by rendering all the ra3 r s of light which 

 enter the eye convergent ; and, therefore, such an individual will not be 

 able to see distinctly without a convex lens in front of the eye. In such 

 an eye the far point is negative, while the near point is abnormally 

 distant and the range of accommodation great. In the hj-permetropic 

 eye, consequently, the distance between the retina and the lens is 

 abnormally short. 



When the eye is accommodated for near objects the pupil contracts ; 

 when in accommodation for distant objects it dilates. So, also, when 

 the eye is exposed to a bright light the pupil becomes reduced in size, 

 while, on the other hand, it dilates when the light becomes reduced in 

 intensity. 



Changes in the pupil are accomplished by the action of the muscular 

 fibres of the iris, which by their relaxation and contraction increase or 



diminish the size of the pupil. 



S^ ^V The iris, therefore, fulfills the 



^=s=»,^^ \ function of a diaphragm and 



/ ^~"**** >iiiSi ss= Si - | serves to cut off the circum- 



T " " -~~^5^ fc^r y~-^ „. ferential rays of light, which 



j_ \ mi « — rr_— " ""ir"" otherwise would lead to the 



\ ./**!» production of spherical aberra- 



N^,^ ^y tion. So, also, as it contracts 



Fig. 384.— Hypermetropic Eye. (Landois.) i n a bright light, it serves to 



regulate the amount of rays of 

 light entering the eye, while, further, it to a certain extent supports 

 the action of the ciliary muscle, as is seen in the changes which occur in 

 the size of the pupil during accommodation. 



The iris is supplied with two sets of muscular fibres, the circular or 

 sphincter fibres, which are supplied by the oculo-motor nerve, and the 

 radiating fibres, or the dilator of the pupil, supplied chiefly by the cervical 

 sympathetic and the trigeminus. When the oculo-motor nerve is divided 

 the pupil dilates, owing to the contraction of the dilator fibres, which 

 still preserve their integrity. On the other hand, when the sympathetic 

 is divided in the neck the pupil contracts through the antagonistic 

 action of the sphincter fibres. The contractility of the circular fibres is, 

 nevertheless, the stronger, for if both nerves be stimulated together 

 contraction of the pupil will take place. The radial muscular fibres are 

 especially developed in birds, while they have been claimed to be absent, 

 in many other animals. 



Changes in the size of the pupil fall under the head of reflex actions, 



