902 THE SENSE ORGANS AND COMMON INTEGUMENT OF THE DOG 



glands are found at the entrance to the preputial diverticulum. The hairs are 

 sparsely scattered in improved breeds — indeed, in some cases the skin is almost 

 bare. The long hairs or bristles are arranged usually in groups of three. They 

 are most developed on the neck and back. The bristles about the snout and the 

 chin are sinus hairs. 



The claws and their corium resemble those of the ox, but the bulbs are more 

 prominent and form a greater part of the ground surface; they are also better 

 defined from the sole, which is small. The claws of the accessory digits are more 

 developed and their parts better differentiated than those of the ox. 



The olfactory region is extensive in correlation with the large size of the 

 olfactory bulbs; the mucous membrane here is brown in color. 



THE SENSE ORGANS AND COMMON INTEGUMENT OF THE DOG 



THE EYE 



The posterior margin of the orbit is formed by the orbital ligament, as in the 

 pig. The orbital axes if produced backward would include an angle of about 79°, 

 the optic axes an angle of about 92.5° (Koschel). The lower eyelid has no distinct 

 cilia. The medial angle is wide, the lacus lacrimalis shallow, and the caruncula 

 lacrimalis small and yellowish-brown. The conjunctival epithelium is cylindrical, 

 with many goblet cells. The gland which surrounds the deep end of the cartilage 

 of the third eyelid is large and pink; it is a mixed gland. 



The lacrimal gland is flat and lies chiefly under the orbital ligament. It is 

 pink in color and mixed in structure. The naso-lacrimal duct is variable in arrange- 

 ment and may differ on the two sides. It may open into the ventral meatus im- 

 mediately after its emergence from the osseous lacrimal canal; thence it continues 

 forward and opens on the lateral wall of the nostril below the ventral turbinate fold. 



The eyeball is almost spherical and is relatively large, especially in the small 

 breeds. The sclera is thick in the ciliary region, where it contains a well-developed 

 venous plexus; in the vicinity of the equator it is thin, and the chorioid shows 

 through it. The cornea is almost circular. The chorioid is richly pigmented, 

 and presents a well-defined tapetum cellulosum; this is somewhat triangular in 

 outline and has a metallic luster. Its color is often golden green, shading to blue 

 peripherally, but the blue tone may predominate, and in other cases it is mainly 

 golden yellow or reddish-yellow. The appearance is due to several layers of peculiar 

 flattened polygonal cells. The ciliary muscle is more developed than in the other 

 species. The iris is commonly light or yellow brown, but not rarely has a blue 

 tinge; the color often differs in the two eyes. The pupil is round; its edge is 

 either smooth or bears minute round prominences. The retina presents a round 

 area centralis lateral to the optic papilla. The latter is variable in outline; com- 

 monly it has the form of a triangle with the angles rounded off, but is often round 

 or oval. Branches of the central vessels of the retina radiate from the middle of 

 the papilla. The surfaces of the lens are not so strongly curved as in the other 

 domestic animals, and there is little difference in the radius of curvature of the two 

 surfaces. 



THE EAR 



The external ear differs greatly in size and form in the various breeds; in most 

 it is relatively wide and thin and the greater part of it hangs down over the opening. 



