200 . ELEMENTARY BIOLOGY. [XIII. 



}>. Enlarge the mouth-opening by cutting through 

 the sides of the buccal cavity -with a pair of 

 scissors : pull down the lower jaw so as to see 

 the chamber (pharynx) behind the mouth. 



c. On the floor of the pharynx is a narrow opening 

 (the glottis): pass a probe down it through the 

 larynx and very short trachea into the lungs. 



d. Remove the lungs ; open one ; it is a thin-walled 

 cavity with a sacculated inner surface. 



e. Trace the oesophagus up to the pharynx. 



C. THE CIRCULATION OF THE BLOOD IN THE FROG'S WEB. 



1. Get a piece of thin board, about 5 inches long and 

 2 J broad ; in the middle of one end of it cut a Y-shaped 

 notch about the size of a spread-out frog's web : place 

 the frog on the board, belly downwards, and fix it by 

 passing round it two or three turns of tape: next 

 tie threads round the toes of one hind-foot, and by 

 means of them spread out the web over the notch 

 in the board, taking great care that it is only very 

 lightly stretched. The animal should be kept moist 

 by a bit of wet blotting-paper spread over its back. 



2. Examine the web with 1 inch obj. : Note 



a. The black pigment-cells in the skin; sometimes 

 irregularly branched; sometimes more compact. 



&. The close network of blood-vessels lying deeper 

 than the pigment-cell layer. 



a. The arteries, running mainly towards the free 

 edge of the web, and constantly diminishing 

 m size as they give off branches; the blood- 

 flow in them from larger to smaller branches. 



