CHAPTER VI. 



THE FROG (continued) : the vascular system — the cir- 

 culation OF the blood. 



In our preliminary examination of the frog (Chap. 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 



