112 ELEMENTARY BIOLOGY. [CHAP. 



tition separating the apposed sacs of opposite 

 sides. 

 /?. The boundaries of the olfactory organ. It lies 

 within an area enclosed by the premaxilla and 

 front end of the maxilla and the palatine bones, 

 being wholly in front of the latter. 



c. Obtain a second frog, and lay open the olfactory 

 sac from the side, cutting obliquely from the 

 anterior nostril across the outer anterior border of 

 the eyeball, in order that the line of section may 

 pass through both anterior and posterior nares. 

 a. General. Cf. supra. 



/?. The anterior nostril ; its upper lip, relatively 

 small and constricted ; its lower lip, produced 

 antero-laterally into a deep fold around the ali- 

 nasal cartilage (cf. Sect. G. 5. a. e). 



3. The visual organ. 



i. General. 



Take an uninjured frog and examine its eye. It will 

 be found to project considerably above the top of 

 the head, but if touched it is withdrawn into a sort 

 of socket. If the animal's mouth be opened, a de- 

 pression, caused by the eye-ball, will be seen on its 

 roof, and this is more prominent when the eye is re- 

 tracted. When the eye is open observe from without. 



a. The eyelids ; folds of skin developed round the 

 margin of the eye-ball. The upper lid is thick 

 and swollen, the lower lid thin and transparent. 

 Gently touch the eye and observe that it is 

 closed, by the pulling over it of the lower eye- 

 lid. The upper lid is hardly moveable. 



