306 A Manual of Veterinary Physiology. 



As before noticed, there is no yellow spot in the eye of 

 herbivora, and the tapetum is probably the area of the most 

 acute vision. 



In the rods of the rod and cone layer of the retina is a 

 peculiar pigment known as the visual purple ; it can be ob- 

 tained from eyes kept in the dark, but in those exposed to 

 light it rapidly becomes bleached, though it reappears 

 when the light is again excluded. 



Pictures have been printed on the retina through the aid 

 of the visual purple ; its exact use is not known, though 

 the photographer's process is suggestive of its probable 

 use. Unfortunately for this theory it is absent from the 

 cones, which are known to be the instruments of the most 

 acute vision. 



Movements of the Eyeball. — We have now to consider the 

 mechanism by which the eye is brought to bear on any 

 desired point, and the subject is somewhat complicated by 

 the fact that with most quadrupeds the eyes occupy a 

 lateral position in the head, in such a way that in 

 many cases vision is only one-eyed, and not two-eyed as 

 in the human subject. Further, the chief movements of 

 the head are not from side to side as with us, but up and 

 down, and as the pupil remains horizontal, no matter what 

 the position of the head, it is evident that rotation of the 

 eyeball must occur. If it were not for this rotation the 

 pupil in the uplifted head would be vertical instead of 

 horizontal, and, in the depressed head, it would be obliquely 

 instead of horizontally placed. 



The muscles working the eyeball are seven in number : 

 four recti, two oblique, and one retractor. The use of the 

 recti is clear enough ; they turn the eye in four directions, 

 outwards, inwards, downwards, and upwards. The two 

 oblique muscles rotate the eye in opposite directions; tin.' 

 superior oblique turns the nasal side of the pupil down- 

 wards, and the eye can rotate upwards until the pupil, 

 with the head in the normal position, becomes vertical 

 instead of horizontal; the inferior oblique turns the nasal 

 side of the pupil upwards. The retractor pulls the eye 



