THE PIGEON 185 



has its insertion on the inner surface of the eyeball, will be seen. 

 All these rectus muscles have their origins in the posterior wall of 

 the orbit. 



Lift up the eyeball and cut the inferior rectus muscle and the 

 optic nerve ; remove the eyeball from the orbit. Without disturb- 

 ing any of the structures of the orbit, observe the origins of the 

 two oblique muscles in the anterior part of the orbit, and of the 

 rectus muscles near the optic nerve. Lift up the inferior oblique 

 and find the Harderian gland. 



Two additional muscles, the quadrate and the pyramidal, are 

 present. These lie close to the inner surface of the eyeball and 

 control the nictitating membrane. The quadrate is a broad mus- 

 cle, the origin of which extends from the dorsal margin of the 

 eyeball to the optic nerve. The pyramidal is a narrow, triangular 

 muscle, which extends from the ventral margin to the optic nerve, 

 where it meets the inner border of the quadrate. 



Study the eye itself. The outer coating is the sclerotic layer, 

 as we have already seen. Cut a small piece from the side of the 

 eyeball near the bony ring which surrounds the cornea : the two 

 other coatings of the eye, the choroid layer and the retina, will 

 be seen. The former is the vascular layer ; it contains the blood 

 vessels which supply the eye ; it contains also the pigment. The 

 iris is the continuation of the choroid layer over the front of 

 the eye ; in it are circular and radial muscle fibers, by the action 

 of which the size of the pupil and the amount of light entering 

 the eye are controlled. 



The retina is the soft inner coating of the eye. It is the essential 

 sensory portion of it, and the direct continuation of the optic nerve. 

 Projecting through the retina at the back of the eye is a prominent 

 pigmented and vascular ridged plate called the pecten. It belongs 

 to the choroid layer and extends into the middle of the eye toward 

 the lens, with which it is joined by a transparent ligament. Its 

 function is unknown, although it may be to supply the lens with 

 nutriment. It may be easily observed if a small piece is cut 

 from the side of the eyeball opposite the first opening made. 



Just back of the pupil and the iris is the crystalline lens. It is 

 lenticular in shape, and flatter on the outer than on the inner side. 



