20 



THE FROG 



Exercise 18. — The Eye. 



(b) Remove the skin from the dorsal surface of the head. Take 

 out the eyeball after cutting the muscles by which it is attached to 

 its socket. What is their function? The optic nerve can be recog- 

 nized where it enters the back of the eyeball (Fig. 20). The outer- 

 most layer on the back of the eyeball is the sclera. It is continuous 

 with the transparent cornea in front. Notice the iris with its opening, 

 the pupil. What is the function of the iris? Use your scissors to 

 open the eyeball by a cut from front to back. Study the pieces under 

 water. The lens is just behind the iris and, if not detached by the 

 cutting, will be found attached to the iris. Internally, the eyeball is 



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Fig. 20. — The eye, in section, showing details of structure and also the manner 

 of formation of an image upon the retina; diagrammatic. 



(Adapted from a figure in C. Hill, "Manual of Histology and Organology," copyright, 

 1923, by W. B. Saunders Co., printed by permission.) 



separated into an outer cavity which lies between the lens and the 

 cornea and contains a fluid called the aqueous humor, and an inner 

 cavity which is filled with the vitreous humor. On the inner surface 

 of the sclera, and closely applied to it, is the dark-colored choroid, a 

 layer which is continuous with the iris. The innermost layer, which 

 lines the cavity behind the iris, is the retina. In life the retina is 

 closely applied to the choroid but may become separated from it 

 during dissection. The retina is the layer with which the optic nerve 

 is continuous. Other parts of the eye are devices to protect the 

 retina and to transmit the light rays to it in the form of a sharp 

 image. Make a drawing ( X 10 or 15) of the eyeball as now opened. 

 After completing this drawing, remove the lens, lay it on a printed 



