62 THE BIOLOGY OF THE FROG chap. 



CHAPTER III 



EXTERNAL CHARACTERS OF THE FROG 



It will be convenient to begin our study of the frog by a 

 description of tlie principal external features of its structure. 

 The flattened more or less triangular head is broadly united 

 to the trunk, there being no region that can be properly 

 called a neck. The large eyes commonly protrude consid- 

 erably, but can be withdrawn into the orbits. Press upon 

 one of the eyes with the fingers, and it will be found that it 

 can be forced inward even beyond the general surface of the 

 head. If now the mouth of the frog be opened, it will be 

 seen that there is a marked prominence in the roof due to 

 the fact that the eye is pressed against the membrane lining 

 that portion of the cavity. The orbit or eye socket of the 

 frog, therefore, is not separated from the mouth by any of 

 the bones of the skull, which is a very different condition 

 from what we find, for instance, in ourselves. In the center 

 of the eye is a dark oval opening, the pupil, which is sur- 

 rounded by a brightly colored ring, or iris. The eye, as in 

 ourselves, can be covered by a pair oi eyelids. The upper eyelid, 

 however, is capable of but little movement, but the lower lid, 

 which is thin and more or less transparent, can be drawn up 

 so as to cover nearly the whole eye. It will be noticed that 

 each time the frog closes its eyelids the eye is pressed into 

 the head, a fact which gives the winking of the frog so 

 peculiar an appearance. The lower lid of the frog is not 

 quite the same organ as the lower eyelid of most animals. 



