850 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



which are aquatic, such as the cetaceans, the crystalline lens is almost as 

 spherical as in the fish, and in them, also, the difference in the refrangi- 

 bility of the different media of the eye is much less than in animals living 

 in the air. 



In many mammals, at the base of the eye is found a collection of 

 brilliant pigment-cells, which reflect the rays of light falling upon the 

 retina and so give to these eyes when seen in semi-darkness a peculiar, 

 luminous appearance. This tapetum is yellow in the ox, reddish yellow 

 in the cat, and blue in the horse. 



In mammals the eyes are placed in orbits whose direction is more 

 or less inclined toward the sides. Only in man, apes, and nocturnal 

 birds of prey are the orbits so arranged that vision may be directed 

 forward simultaneously on the two sides. 



The Iachr3 r mal apparatus of mammals is composed of a single or 

 double lachiymal gland placed at the external angle of the orbit. Gar- 

 ni vora, rodents, pachyderms, and 

 some ruminants have in addition in 

 the internal angle of the orbital cav- 

 ity the so-called gland of Harder, 



FIG. 368. FORMATION OF AN IMAGE BY 

 REFLECTION. ( Ganot. ) 



The rays from the object A B are reflected by the 

 mirror N M so as to make their angle of reflection equal 

 to their angle of incidence; if a perpendicular, A D, is 

 let fall from the point, A, and one, B C, from the 

 point, B, and the reflected rays prolonged until they 

 meet these perpendiculars, the image is apparently 

 formed behind the mirror at a distance equal to the 

 actual distance of the object in front of the mirror. 



FIG. 369. DIAGRAM ILLUSTRATING RE- 

 FRACTION OF LIGHT. (Ganot.) 



The incident ray, S O, on striking the surface, n in, 

 is bent in the direction OH. SAO being the angle of 

 incidence, HOB the angle of refraction. 



which furnishes a thick, whitish secretion, which often accumulates at 

 the corresponding angle of the eyelids. Rudiments of this gland are 

 also found in solipedes. The tears are collected by the lachrymal points, 

 which conduct them through the lachrymal duct and nasal canal to the 

 nasal cavities.. In certain rodents, the hares in particular, the lachrymal 

 canals are replaced by fissures which establish communication between 

 the conjnnctival surface and the nasal fossae. 



In the study of the appreciation of the impression of a ray of light 

 upon the retina and the formation of an image a comprehension of the 

 laws of light is essential. The eye is furnished with certain mechanisms 

 by which an image is formed somewhat in the same manner as in a 

 camera obscura. Such mechanisms are what are termed the dioptric 

 mechanisms of the eye. Rays of light striking the 'retina give rise to 



