DISSECTION OF A FROG. 41 



In the hind leg do you find any parts besides thigh, 

 shank, ankle, instep, and toes ? If you have any difficulty, 

 compare the way in which the joints bend with those in 

 your own body, and find where your trouble is. 



INTERNAL STRUCTURE. 



Beginning just in front of the vent, slit the skin of the 

 ventral surface in the middle line forward to a point be- 

 tween the shoulders. Turn back the skin on either side. 

 Is it firmly attached to the underlying muscles ? Are there 

 blood vessels on its inner surface ? Notice the muscles ; 

 can you find any muscle -plates (p. 11) ? 



Next cut the muscles in the same way, a little to the 

 (animal s) left of the middle line, carrying the incision for- 

 ward through the hard parts between the shoulders, and 

 taking great care to keep the underlying parts uninjured. 

 This lays open the peritoneal cavity (ccelom). Insert a blow- 

 pipe into the gullet and inflate the stomach. Is there any 

 sharp boundary between it and the intestine ? Is the in- 

 testine more or less coiled than in fish or dogfish ? Is it 

 of the same size throughout ? How is it suspended ? 



Does the liver cover the stomach ? Turn the liver for- 

 ward and look for the greenish, spherical gall-bladder and 

 the light-colored, lobulated pancreas. Do you find ducts 

 from either of these to the intestine ? Farther back, in 

 the mesentery, near the enlarged portion (rectum) of the 

 intestine, is the. red spleen. At the posterior portion of the 

 peritoneal cavity is the thin-walled urinary bladder. With 

 what is it connected ? 



Draw the digestive organs, showing the position of the 

 deeper structures by dotted lines. 



Turn the intestines, etc., out of the body, exposing the 



