3 8 THE FROG 



where the artery leaves the body cavity by thrusting a pin 

 through the body wall to the dorsal surface. Remove the 

 skin from the dorsal surface of the body and hind limb; 

 dissect away the muscle and connective tissue posterior to 

 the point indicated by the pin and expose the sciatic artery. 

 Follow the sciatic artery down the dorsal side of the thigh 

 by forcing apart the great muscles in this region. 



3. The pulmo-cutaneous arch, or pulmo-cutaneous ar- 

 tery, is the most posterior of the three aortic arches. A 

 short distance from its origin it divides into two branches, 

 as follows : 



The pulmonary artery passes straight to the lung. At 

 the root of the lung it divides into a number of branches 

 which run toward the tip of the lung, each giving off 

 branches to the interior trabeculae. 



The cutaneous artery at first turns laterally, slightly for- 

 ward and dorsalward, crossing the systemic arch; it then 

 turns sharply dorsalward. To trace its further course turn 

 the frog on its ventral surface, turn back the flaps of skin 

 on the dorsal surface, and carefully dissect away the muscle 

 just posterior to the tympanum. The cutaneous artery will 

 then be seen emerging from the body cavity just in front of 

 the suprascapula. (Since the suprascapula of one side has 

 been removed, it may be necessary to work on the opposite 

 side.) Here it divides into three branches; trace their dis- 

 tribution. What evidence do you find that the skin is an im- 

 portant organ of respiration? 



Compare the arterial system of the adult frog with that of 

 the tadpole (see Leuckart's chart, or Bourne, Comparative 

 Anatomy, Vol. 2, Fig. 68). What changes are required in 

 the substitution of pulmonary respiration for breathing by 



