28 APPLIED BIOLOGY 



notice : the thick fleshy thigh, extending from the hip to the knee ; 

 the shank, extending from knee to ankle ; and the foot. How many 

 toes? Compare with number of "fingers." The shortest toe 

 corresponds to the big toe of the human foot. 



Notice the membrane ("web") stretched between the toes of the 

 hind foot, fitting the foot as a paddle for swimming. When you 

 have an opportunity to observe frogs swimming in a large aquarium, 

 or in the clear water of a pond, compare the uses of fore and hind 

 limbs. The fitting of any structure or organ of an animal or plant 

 to a special function is known as adaptation. Another adaptation 



of the frog's hind legs is in the 

 great muscles of the thigh, 

 which fit it for jumping. 



Mouth. The opening is 

 the mouth, and inside is the 

 mouth-cavity (also called buc- 

 cal cavity, meaning cheek 

 cavity). Commonly we speak 



t* *~^ x tlM^sfi^^^^ ^~L~- ^ *ke numan mouth-cavity 



" " " as mouth, and the opening 



as lips ; but to be accurate 

 FIG 3. Various positions taken by tongue mouth should be applied 

 of frog when catchmg an insect. (From h O p e ning. Examine the 

 Holmes, after Wiederscheim.) 



mouth-cavity of a frog which 



has been chloroformed. Are there teeth? Look for teeth on a 

 mounted skeleton of a frog. Notice two small openings (nostrils) on 

 the dorsal side of the head near the upper lip of the frog's mouth. 

 Pass a shoemaker's bristle or a slender broom-straw into a nostril, and 

 note where the bristle comes out into the mouth-cavity. The human 

 nostrils have no such direct communication with the mouth-cavity. 

 With a needle make a hole in the frog's ear-membrane back of the 

 eyes, and then carefully push a bristle into the opening. Open the 

 mouth and find where the bristle comes out into the mouth-cavity. 



To prove a similar connection between the human ear and the 

 back part of the mouth-cavity, close your nostrils with your hand, 

 then swallow once or twice and notice a feeling of pressure in your 

 ears, due to forcing air back into the internal part of the ears. This 

 explains why workmen are told to swallow air when entering the 

 compressed-air chambers used in tunneling under rivers. The tubes 

 connecting the internal parts of the ears with the mouth are known 

 as Eustachian tubes, named in honor of Eustachius, professor of 

 anatomy at Rome, who wrote a book describing the ear in 1754. 



