SPECIAL SENSES 107 



does its contraction take place? What motion does it give 

 to the eye? On the under side of the eye locate another 

 transverse muscle (the inferior oblique). Where is its ori- 

 gin? Has it a pulley? 



B. The Externals of the Eye. Cut the muscle bands and 

 trim away a white membrane (the conjunctiva, a continua- 

 tion of the lining of the eyelid) in the front of the eye. Note 

 that the eye is still attached to the socket by a cord, just 

 below and outside the center of its rear surf ace. This is the 

 optic nerve , which enters the eye here from the brain. Pull 

 the eye out of the socket and cut this cord. Now examine 

 the outside of the eyeball. Note that it is covered with a 

 firm white coat (the sclerotic) except in the front, where 

 there is a clear layer, the cornea, usually dulled in death. 



C. The Internals of the Eye. Hold the eye with the cornea 

 uppermost, and remove this with the scalpel by cutting hori- 

 zontally around its edge. The liquid back of this layer is 

 the aqueous humor. Directly back of the cornea appears a 

 circular muscular curtain colored in the human eye called 

 the iris, and in its center a hole the pupil. What conclu- 

 sions do you draw as to the functions of this iris from compar- 

 ing the size of the pupil of your own eye, when looking at a 

 bright light, with its size when in a dimly lighted room? 

 Is its action voluntary? 



Now lay the eye upon its side in the evaporating dish 

 and cover it with water. With the scalpel cut a section 

 through the entire eye, splitting the optic nerve (see Fig. 

 51). Observe the following parts: just back of the iris 

 the convex crystalline lens and its capsule ; the muscles that 

 control the shape of the lens the ciliary muscles and their 

 ligamentous attachment (suspensory ligament) ; on the inside 

 of the layers that form the walls of the eye, at the edge of 



