188 VERTEBRATES. 



by their union is continued posteriorly into the midbrain, 

 and expands there into a narrow cavity, extending ob- 

 liquely upward and downward, the third ventricle. A 

 narrow, median canal, the iter, connects the third and 

 fourth ventricles. A pair of round cavities, the optic 

 ventricles, occupy the optic lobes, and are connected with 

 the iter. If the brain be turned to one side, and drawn 

 upward out of the cranium a little, there may be seen a 

 downward prolongation from the midbrain toward the 

 roof of the mouth. It is the pituitary body. 



If the foregoing points in the study of the brain have 

 not been made out satisfactorily in a fresh specimen, place 

 a frog's head a few days in alcohol to harden, and with it 

 try again. 



The Ear. In the ear observe the tympanum, from 

 which the skin has been removed. Observe the opaque 

 spot in it, and the surrounding transparent area. The 

 opaque spot is the point of attachment of an elastic rod 

 which extends across the cavity inside, and communicates 

 with the inner ear. When sound waves fall on the tym- 

 panum, and cause it to vibrate, its vibrations are carried 

 through the elastic rod to the inner ear, which is the true 

 organ of hearing, and which is securely lodged within the 

 bone of the skull. Dissect out this elastic rod. It is 

 called the columella. It is sometimes partly ossified in 

 the middle. 



Appendages. That there are two pairs of legs will 

 have been noticed already. Which pair is most service- 

 able in locomotion? What peculiarity in its structure 

 would indicate that a frog could jump well, were its 

 habits entirely unknown? What other animals have you 

 seen having the same peculiarity of structure, and corre- 

 sponding ability to jump ? 



In fore and in hind leg find three principal divisions, 



