THE FROG 75 



Dissect the frog under water in the pan. Lay the body 

 on its back, stretch out the legs and pin them down. Then 

 cut through the skin on the median line of the ventral sur- 

 face from the lower jaw to the posterior end of the body. 

 Make a transverse cut across the middle and turn back the 

 four flaps and pin them down. If the skin has been care- 

 fully cut, the thin abdominal wall will now appear unharmed. 

 Note a dark vein along the median line showing through the 

 wall. Cut through the wall to one side of the vein so as not 

 to injure it, and continue the slit to the breastbone. Raise 

 the breastbone and see the heart. Then cut through this 

 bone a little to one side, and pin the flaps of the abdominal 

 wall out of the way to expose the internal organs. 



The most conspicuous organ is the reddish liver. How 

 many lobes has it ? Find the gall bladder, a dark, spherical 

 sac, on the under side of the liver. Trace its connection 

 with the intestine. 



Circulatory organs. Just in front of the liver is the heart, 

 inclosed in a thin, transparent sac, the pericardium. Pinch 

 up the loose pericardium and cut through it, being careful 

 not to sever any of the blood vessels, and remove as much 

 of it as possible. Note the heart inside, with two auricles 

 at the base and the single ventricle at the posterior end. 

 If the heart is still beating, it may be possible to time its 

 pulsations. 



Note the large artery that springs from the anterior end 

 of the ventricle. It soon divides into two main arteries, 

 each of which then divides into three others, called the aortic 

 arches. The anterior arch carries blood to the head; the 

 middle arch carries blood to various parts of the body; 

 while the posterior arch conveys blood to the lungs and 

 skin for aeration. 



