AN INTRODUCTION TO ANIMAL BIOLOGY 31 



smaller branches leading to the right and left * sides of the body. 

 These are the chief blood-tubes through which blood flows away 

 from the heart, and they are called arteries. The branches of the 

 two arteries lead to all parts of the body, and may be seen in many 



FIG. 5. Frog dissected from ventral surface, h, heart; Ig, lungs; I, liver; 

 m, stomach ; d, intestine ; e, ovaries ; 6, bladder. (From *Ecker.) 



organs. Connecting with the anterior end of the heart and on its 

 dorsal surface are three thin-walled tubes, which, in dead frogs, are 

 usually filled with dark-purple blood. These are the blood-tubes 

 in which blood flows back to the heart, and they are called veins. 



* The student should remember that with the frog lying on its back the 

 right side of the frog will be on the observer's left. In all descriptions in this 

 book, right and left refer to the animal studied, not to the observer. 



