THE FROG 83 



body, the vitreous humor. In preserved material the vitre- 

 ous humor may not be readily apparent : it may be shrunken 

 and fused indistinguishably with the retina. 



4. The choroid membrane is the black pigmented layer 

 lining the sclerotic and continuous in front with the iris. 

 What is its function? Why is the pupil of the eye of the 

 living frog black? 



5. The retina is a delicate membrane whose sensitive por- 

 tion lines the eye-ball within the choroid membrane, and is 

 thickest in the proximal region of the eye-ball. In life the 

 sensitive portion of the retina is nearly transparent, slightly 

 grayish; in preserved material it is yellowish-white. In 

 fresh material, the retina is marked on its inner surface by 

 a distinct white spot, the "blind spot," at the point where 

 the optic nerve enters. The retina is readily detached from 

 the choroid, except at the point where the optic nerve enters. 



Using the outline already made, construct a diagram 

 showing the structure of the frog's eye in a proximo-distal 

 section passing through the pupil and the optic nerve. 



XV. THE EAR. 



Cut off the head of a frog (the specimen used for general 

 dissection will do) just posterior to the tympanum, and de- 

 calcify the skull by immersing the entire head for several 

 days in 50 per cent, alcohol containing 10 per cent, nitric 

 acid. Remove the skin and observe the tympanic mem- 

 brane or drum membrane of the ear (transparent after the 

 removal of its outer or tegumentary layer), supported at its 

 edges by a cartilaginous ring. To the center of the inner 

 surface of the drum membrane is attached the distal end of 

 a small bony and cartilaginous rod, the columella, visible 

 through the tympanic membrane. Tear off the tympanic 



