THE NERVOUS SYSTEM 39 



the attached end which is moved when the muscle con- 

 tracts is called the insertion. The superior oblique muscle 

 has its origin at the anterior wall of the orbit. The other 

 three are attached to the lateral posterior wall ; the most 

 anterior of these being the internal rectus with its insertion 

 on the eyeball covered by that of the superior oblique; 

 most dorsally the superior rectus ; and the one on the pos- 

 terior side, the external rectus. Determine what would be 

 the result of the contraction of each of these muscles. 



Raise the eyeball and cut through the conjunctiva, a 

 thin layer adhering to the external surface of the eyeball 

 and continuous with the lining of the lower lid. Lift the 

 eyeball and notice the inferior oblique muscle originating 

 from the anteriomedial corner of the orbit and the inferior 

 rectus from the posteriomedial angle of the orbit. They 

 are inserted together in the middle of the ventral face of 

 the eyeball. 



Drawing 21. Draw the eyeball showing muscles. 



Remove the eyeball by cutting through the eye muscles 

 at their insertions. The outermost coat covering the front 

 of the eyeball is the conjunctiva which is deflected onto the 

 inner surface of the eyelids. It is a part of the epidermis 

 of the skin and not a true coat of the eye. The outermost 

 coat of the eye ball is the sclera, a tough membrane of con- 

 nective tissue. It is transparent in front, forming the 

 cornea to which the conjunctiva is inseparably fused. Cut 

 off the dorsal side of the eyeball and study under water. 

 Note the crystalline lens which aids in focusing the light, 

 the black choroid coat internal to the sclera which darkens 

 the interior of the eyeball, and internal to the choroid coat 

 is the greenish layer, the retina which is often collapsed. 

 The retina is an outpushing of the brain containing the rods 

 and cones which can be stimulated by the light. Follow 



