Xiii.] THE FROG. 249 



sphenoid bone (D. c. 3.0); the chamber in the 

 pro-otic bone (D. c. i. d) which is thus exposed 

 contains part of the internal ear. 



c. The dissection of the internal ear of the frog 

 is difficult, on account of its minuteness. But 

 by carefully removing the bony and cartilagi- 

 nous walls of the periotic capsule, piecemeal, 

 the semicircular canals will be exposed and the 

 whole membranous labyrinth may be extracted. 

 It should be placed in a watch-glass containing 

 salt solution or spirit, and its form studied under 

 the simple microscope. 



c. The olfactory organs. 



i. These consist of two chambers which open externally, 

 near the end of the snout, by the anterior nares, and 

 posteriorly into the mouth, just behind the vomerine 

 teeth by the posterior nares. Make out these open- 

 ings. 



a. Take a frog which has been preserved in spirit 

 and pass the point of a small pair of scissors into 

 the external nostril of one side and cut away the 

 roof of the nasal cavity. A chamber is thus ex- 

 posed which has a somewhat triangular form, the 

 apex of the triangle being at the external nostril 

 and the posterior nostril being at another angle 

 and farther from the middle line. 



b. The walls of the cavity are slightly folded, and 

 there is a well-marked hemispherical eminence 

 on its floor. 



c. Open the other nasal cavity in a similar way: 

 notice the boundary wall (septum narium) which 



