CHAPTER VI. 



THE FROG (continued] : THE VASCULAR SYSTEM THE CIR- 

 CULATION OF THE BLOOD. 



IN our preliminary examination of the frog (Chapter II) 

 we learned one or two facts about the vascular system. We 

 found that there is a heart within a pericardium, two sets of 

 vessels, arteries and veins, containing red blood, and a set 

 of irregular cavities or sinuses, containing lymph. We must 

 now try to get some more accurate and detailed information 

 on these matters. 



General Characteristics of Blood and Lymph. It will 

 be convenient to begin by studying certain easily verified 

 characteristics of the blood. Frog's blood may be used, or 

 as it is as well to have a considerable quantity, that of some 

 larger, freshly-killed, red-blooded animal, such as a rat or 

 rabbit. 



When first drawn from the heart or vessels the blood will 

 be seen to be a fluid, nearly as mobile as water or milk ; 

 it " finds its level," like any other liquid, and can be readily 

 poured from one vessel to another. In a few minutes, 

 however, it undergoes a change ; it ceases to be fluid, and 

 coagulates, or " sets " into a jelly, which if turned out of the 

 vessel, retains the shape of the latter. Before long a further 

 change takes place ; the jelly begins to shrink, drops of 



