Amphibia. 85 



near the pelvis. With scissors snip through the skin in the middle 

 line. Holding the edge of the skin with the forceps, slit the skin 

 from the pelvis to the chin. Loosen the skin wherever it adheres 

 to the underlying tissues, and turn it back. Cut outward in the 

 middle of each flap of skin, and pin out if necessary to keep it out 

 of the way. Now cut through the muscular wall of the abdomen in 

 the same way, but be careful to keep to one side of the middle line, 

 and thus avoid cutting the central blood tube. Be also especially 

 careful to lift the edge of the cut with the forceps, and watch closely 

 the lower point of the scissors to see that none of the abdominal 

 organs are injured. When the breastbone is reached, raise it to 

 see the heart. Now cut through the breastbone a little to one side. 

 Loosen the tacks in the hands and set them farther out to expose 

 the organs ; tack out the flaps of the abdominal wall. Keep the 

 specimen covered with water ; renew the water if it becomes 

 turbid. 



2. Covering most of the internal organs is the dark liver, con- 

 sisting of several lobes ; note how many lobes there are and how 

 they are arranged. Slip the handle of the scalpel under the pos- 

 terior border of the liver and tip it forward. Observe the bile sac, 

 a dark, usually bluish, spherical sac, connected with the liver. 



3. Between the lobes of the liver at its anterior edge is the 

 reddish heart. It is inclosed in a very thin sac called the peri- 

 cardium. Pinch this up with the forceps, cut through it, then 

 remove most of it. The heart consists of two auricles at the base, 

 and the single ventricle at the apex, or posterior end. In a freshly 

 killed frog the heart often may be seen beating. Time its pulsa- 

 tions. Running forward from the ventricle is the main artery. 

 This divides into two branches, right and left, each of which has 

 three subdivisions : 



a. To the head, the carotid artery. 



b. To the body generally, the aorta. The two aortas unite in 

 the posterior dorsal part of the body cavity. 



c. To the lungs and skin, the pulmo-cutaneous. 



What is the effect of applying gentle heat to the heart, as by 



