VETERINARY OPHTHALMOLOGY. 15 



suit. The sphincter fibers contract and narrow down 

 the pupil. If both nerves be stimulated simultane- 

 ously, we will observe that the sphincter set are the 

 more powerful, for contraction will ensue. In the pre- 

 sence of bright light we have contraction. Stimulation 

 in the floor of Aqueduct of Sylvius causes contrac- 

 tion. Pupillary caliber is modified by action of 

 certain drugs, of which more hereafter. The retinal 

 action and its results are far from being satisfactorily 

 explained to the ophthalmic student, as yet. We know 

 that it is the rod and cone layer only, which is con- 

 cerned in the formation of the image. The most acute 

 vision is at the macula lutea, or yellow spot. We 

 will speak of only the rod and cone layer in this part 

 of our little brochure, and under the retina will delve 

 deeper into its layers. 



The external layer, consisting of rods and cones 

 closely packed together, *'. e., small transparent rods, 

 end on, close together, and scattered among them with- 

 out regularity, a cone here and there. At the macula 

 we find numerous cones and an absence of rods. Also 

 at this place, find ganglionic and yellow pigment cells. 

 Now, remember, light is a sensation only. 



Remember, also, we spoke of the visual purple. As 

 yet we know not of its i^recise function, but that it is 

 concerned in the perception and recognition of light, 

 there is no reasonable doubt. 



The movements of the bulbus in its socket are of 



