THE PERCH 207 



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 

 movements 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 in- 

 sertion of the external 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 rectus, which runs back to the inner posterior 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 9. Draw a sketch of the orbit showing its muscles and the 

 optic nerve. 



Study the eyeball. Its wall is composed of three coats, the 

 sclera, the choroid, and the retina. The tough outer covering is 

 the sclerotic coat, or the sclera, of which the cornea is the portion 

 in front. Cut the eyeball in two lateral halves ; remove the other 

 eye and cut it in an anterior and a posterior half ; study the in- 

 terior of both under water. Just back of the pupil is the spherical 

 crystalline lens, held in place by the iris ; do not remove it. This 

 is the shape of the lens in all vertebrates which live under water. 

 In a dense medium like water vision is necessarily limited in 



