42 VERTEBRATE ZOOLOGY 



pupil ; and the transparent cornea, which lies in front of both and 

 forms the outer coating of the eye. Observe carefully the circular 

 fold of the skin which surrounds the front of the eyeball. 



With strong scissors cut away the circular ridge of the skull 

 which surrounds the eye, and remove the muscles of the head just 

 beneath and back of it. Remove also the slimy fold of the skin 

 just mentioned, which surrounds the front of the eye. Note that 

 a transparent layer of the skin passes over the cornea and may be 

 peeled off ; this is the conjunctiva. 



Observe the position of the eye in the orbit ; it is held in place 

 by the optic nerve, which joins it at its inner end, and by six small 

 muscles. These muscles have their origin in the wall of the orbit, 

 and their insertion in the outer coating of the eyeball, the move- 

 ments of which they control. 



Study these muscles. Press the eyeball downward and note on 

 its medial side the insertions of two muscles : the anterior one is 

 the superior oblique, which goes from the eyeball to the inner 

 anterior wall of the orbit ; the posterior one is the superior rectus, 

 which goes from the eyeball to the inner posterior wall of the 

 orbit. Push the eyeball backward and note the inferior oblique 

 muscle, which has its insertion on its anteroventral surface and 

 passes parallel with the superior oblique to the inner anterior wall 

 of the orbit. On the posterior side of the eyeball is the insertion 

 of the external (posterior) rectus muscle, which runs to the inner 

 posterior wall of the orbit. Cut the superior oblique muscle at its 

 insertion in the eyeball ; beneath it will be seen the insertion of 

 the internal (anterior) rectus, which runs back to the inner pos- 

 terior wall of the orbit. Cut all these muscles at their insertion in 

 the eyeball and pull it gently outward and forward ; the inferior 

 rectus will be seen, whose insertion is on the inner side of the 

 eyeball and which runs to the inner posterior wall of the orbit. 

 Cut this muscle and the optic nerve and remove the eyeball from 

 the orbit. Note the origins of the four rectus muscles in the 

 posterior wall of the orbit, and of the two oblique muscles in the 

 forward wall of the orbit. 



Exercise 10. Draw a sketch of the orbit, showing its muscles and the 

 optic nerve. 



