The Senses. 



311 



horse, corresponds with the temporal side of the opposite 

 eye ; while the nasal side cannot correspond with the nasal 

 side of the other eye, as it is not possible for a ray of light 

 from an object to strike both nasal sides at one time (see 

 Fig. 36). 



Cartilago Nictitans.— The retractor muscle of the eye with- 

 draws the eyeball within the orbit, and the pressure thus 

 produced within the cavity forces the cartilago nictitans 



Fig. 36. — Diagram of Horizontal Section of the Head passing 

 through both eyeballs, to illustrate corresponding 

 Points in the Retina of the Horse. 



x x, the frontal bones ; p p, portion of malar bone entering into the 

 formation of the outer rim of the orbit ; s, the nasal septum. 

 Rays of light proceeding from A are seen by both eyes, being 

 imprinted on the temporal side of each retina at a ; rays from B 

 are seen at b in the left eye, but are not seen with the right eye. 

 In the same way rays from C are imprinted at c in the left eye, 

 but cannot be seen with the right eye. 



forward to such an extent, that it may be made to sweep 

 nearly the whole corneal surface. 



The reason why the cartilage is pressed forwards is due 

 to the fact that, naturally curved, it becomes flattened by 

 the pressure caused by retraction, and shoots forward ; 

 when the pressure is removed it retires through its own 

 elasticity, becoming curved once more. 



