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 cortum 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° (IXoschel).. 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. 
