THE PERCH 43 



Study the eyeball. Its 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 interior of both under 

 water. Just back of the pupil is the spherical crystalline lens; 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 neces- 

 sarily limited in range, and fishes can see only objects which are 

 close to them. An eye with a spherical lens is shortsighted. 



The inner coating of the eye is the retina. Between it and the 

 sclerotic coat is the choroid coat. This consists of three layers : 

 the black pigment layer, which lies just beneath the retina ; the 

 glistening silvery layer, which is just within the sclera ; and the 

 vascular layer, which contains blood vessels. Around the entrance 

 of the optic nerve the vascular layer is much thickened. The part 

 of the choroid extending over the front of the eye is the iris, the 

 central opening of which is the pupil ; delicate muscles in the iris 

 control the size of the pupil. Around the inner side of the iris a 

 ridge called the ciliary process extends to the lens and is continu- 

 ous with the delicate membrane in which the lens is suspended. 



Note the blind spot, the point where the optic nerve enters the 

 eye. From near this spot a slender projection of the choroid, 

 called the falciform process, which ends in an enlargement called 

 the campanula Halleri, extends through the retina to the side of 

 the lens, to which it is attached. The campanula contains smooth 

 muscle fibers by the action of which the position of the lens is 

 slightly changed and its focus altered. The range of accommoda- 

 tion is, however, very small. Use the hand lens in studying the 

 falciform process ; it is usually easily found. 



The two large chambers of the eye are the one between the 

 iris and the cornea, filled with the watery aqueous humor, and 

 that between the retina and the lens, filled with the jellylike 

 vitreous humor. 



Exercise 11. Draw a diagram representing the structure of the eye. 



The Ear. The auditory organ consists of the membranous laby- 

 rinth alone, which is embedded in the cranium back of the eye ; no 



