THE FROG. 91 



a. To the head, the carotid. 



k. To the body generally, the aorta. Trace the two 



aortse, right and left, to their point of union near 



the spleen. 

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



Part of the impure blood is sent to the skin, through 

 which it gets oxygen. In this way the frog gets 

 oxygen when under water for a long time, and during 

 the winter, when it hibernates deep down in the mud. 

 Remove the heart. Does it beat after removal from 

 the body? Note the effect of holding the heart in 

 the palm of the hand. Note the effect of pricking it 

 with a needle. 



CIRCULATION OF BLOOD IN THE WEB OF A FROG'S FOOT. 



Split a cigar-box cover or shingle in the middle, and cut 

 a Y-shaped notch in one end; wrap the frog in a wet 

 cloth, with one leg projecting, and tie it thus wrapped to 

 the board; tie threads around two of the toes, and stretch 

 the web (but not too tightly) over the V-shaped notch ; 

 place the board firmly on the stage of the microscope. 

 Examine first with a one-inch objective. The large ves- 

 sels which grow r smaller by subdivision are the arteries. 

 The large vessels which are formed by the union of smaller 

 ones are the veins. The finer vessels forming a network 

 are the capillaries; the black spots are pigment cells. 

 Where these are very abundant, they make dark spots, as 

 seen on the frog's back. Take a triangular piece of cover- 

 glass a little smaller than the web under examination; 

 place a small drop of water on one side of it, and lay the 

 glass, with the water downwards, on the web. Put on a 



