14 GENERAL ANATOMY OF THE FROG. 
and at different times: mottled on the dorsal surface, paler on 
the ventral. 
4, The head: flat and triangular, with a blunt apex directed 
forwards. 
At the sides of the head are the eyes, which are large and 
prominent. Each eye has two eyelids of which the upper is thick, 
pigmented, and almost immoveable, while the lower is semi- 
transparent and freely moveable. 
Behind the eye on either side is an obliquely placed elongated 
patch of a dark colour, in the middle of which is a circular 
area—the tympanic membrane—supported by a firm marginal 
ring. 
5. The limbs: two pairs, fore and hind, each composed of 
three segments— 
a. Fore limb: 
i, Arm. 
ii, Forearm. 
iii, Hand, with four digits corresponding to the four 
fingers of man; the thumb being very small and 
inconspicuous. In the male frog at the breeding 
season there is a thickening along the inner edge 
of the first digit. 
b. Hind limb: much longer than the fore limb: 
composed of :— 
i, Thigh. 
i. Leg. 
iii. Foot, with five toes, webbed together. The 
. shortest toe corresponds to the big toe of man, 
and the longest to his fourth toe. 
6. External apertures: or openings on the surface of the body. 
a. Median apertures. 
i, Mouth: a wide horizontal slit. 
ii, Cloacal aperture: a small hole at the posterior end 
of the body between the legs: placed slightly 
on the dorsal surface, just behind the bony pro- 
jection formed by the posterior end of the 
urostyle. 
b. Paired apertures. 
i. Nostrils or anterior nares: two small openings on 
