XV THE CIRCULATORY SYSTEM 279 



wall of the bulbus against the free edge of the spiral valve 

 and prevents the blood from flowing over into the left or pul- 

 monary side of this division of the heart. The blood is pre- 

 vented from access to this side anteriorly by valves, so there 

 is now no course open to it but through the carotid and 

 systemic arches. Since the common opening of the pulmo- 

 cutaneous arches lies behind the valves at the anterior end 

 of the bulbus, it can receive no blood when the communica- 

 tion between the two sides of the bulbus is cut off. In this 

 way the impure blood first sent out of the heart goes mainly 

 to the lungs and skin, where it is purified, while the purer 

 blood passing out toward the close of the contraction of the 

 heart is sent to the various other parts of the body. 



The heart of the frog may beat for hours, or, under fav- 

 orable conditions, even for days, after it has been removed 

 from the body. Even isolated parts of the heart, such as 

 the sinus venosus, auricles, or ventricle, may continue beat- 

 ing, although not with the same rhythm. If the heart is 

 removed so as to leave the sinus venosus within the body, 

 the auricles and ventricle beat with a rate less than the nor- 

 mal, but the sinus continues to beat with nearly the same 

 rhythm as before. If the sinus is removed with the rest of 

 the heart, the beating of the whole heart is more rapid than 

 that of the auricles and ventricle when removed alone. It 

 is apparently the sinus venosus which sets the rhythm for the 

 beating of the other parts of the heart. After the heart has 

 ceased to beat spontaneously it may be caused to resume 

 its activity by the application of a stimulus. 



Circulation in the Web of the Foot. —The web of the 

 frog's foot affords a classical object for the study of the 

 capillary circulation. It may easily be prepared for observa- 

 tion with the microscope by tying the frog down to a small 

 piece of board, and spreading its toes apart so that the web 



