The Eye

Sense Organs and Integument of the Dog

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 inner 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 arrangement and may differ on the two sides. It may open into the inferior meatus immediately after its emergence from the osseous lacrimal canal; thence it continues forward and opens on the lateral wall of the nostril below the inferior turbinal 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 external to the optic papilla. The latter is variable in outline; commonly 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.