PHYSIOLOGY - 185 



lid, secrete tears, This is a salty liquid which lubricates and washes 

 the surface of the eyeball and inner surface of the lids. Seven 

 muscles control the movements of the eyeball. These muscles take 

 their origins on the inner surface of the orbit and have their inser- 

 tions on the surface of the eyeball. These seven include four straight 

 ones (recti), two oblique ones, and one retractor: superior rectus, 

 mesial (internal) rectus, inferior rectus, lateral (external) rectus, 

 superior oblique, inferior oblique, and retractor oculi. The four recti 

 muscles each pull the eyeball in the direction indicated by the 

 name ; for example, the superior rectus moves the eyeball upward, 

 and the inferior one moves it do^vnward. The action of the superior 

 oblique muscle is modified by the fact that it pulls through a pulley 

 which results in a twisting or rotating movement for the eyeball. 

 The inferior oblique opposes it by direct attachment at the opposite 

 position. The retractor oculi originates on the boundary of optic 

 foramen and divides into four heads which are inserted in the 

 sclerotic coat (sclera) near the entrance of the optic nerve. This 

 muscle is almost covered by the recti muscles. 



The eyeball is composed of three tissue layers, a lens, and two 

 humors. The outer, firm sclera is almost white in color and continues 

 forward into the transparent cornea which covers this surface. In- 

 side the sclera is the choroid coat which is generally black in color. 

 On the inside surface is an irregular area which has a metallic luster 

 and is called the tapetum. The choroid extends forward as the iris 

 or colored portion in human beings, but in albino rats the pigment is 

 lacking and it is pink, due to the color of the blood in its tissues. The 

 iris is .incomplete at its forward side, and the aperture in its center is 

 the pupil. Light is admitted through this circular opening. Its size 

 may be adjusted by contraction or expansion of the iris, and in this 

 way the amount of light and divergence of rays are regulated. The 

 inner layer, lining the inner surface of the posterior three-fourths of 

 the eye, is the retina. It is a thin gray membrane and contains the 

 sensory cells (rods and cones), which are connected to nerve fibers 

 coming in by way of the optic nerve which enters the back side of the 

 eye. The convex lens, which is held in position just behind the iris 

 by the ciliary ligament, is crystalline and focuses the light rays on 

 the retina. The area of the retina directly behind the lens is called 

 the fovea. This is the most sensitive part of the retina. Slightly 

 medial and below this is the entrance of the optic nerve, at which 

 point the retina is devoid of rods and cones. The very descriptive 



