THE FROG 59 
cornea and the lens; it is small, and in life contains a watery 
fluid, the aqueous humor. 
3. The posterior chamber of the eye is a large space behind 
the lens; in life it is filled with a grayish gelatinous body, the 
vitreous humor. In preserved material the vitreous humor may 
not be readily apparent: it may be shrunken and fused indis- 
tinguishably 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 portion 
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. Label all the parts. 
OV Ile = Vee Ear: 
Cut off the head of a frog (the specimen used for general dis- 
section will do) just posterior to the tympanum, and decalcify 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 membrane 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: 
membrane without disturbing the columella, and observe a large 
cavity, the tympanic cavity (corresponding to the middle ear of 
man and other mammals), which communicates with the mouth 
cavity by means of the Eustachian tube. Clean away the muscles 
5 
