Circulation of the Blood. 145 



that taken singly, they are but slightly tinged with 

 colour, and it requires the presence of several layers 

 of them to exhibit the well-known redness. A single 

 corpuscle, however, when seen edgewise, if it be a 

 large one, as that of a frog, will exhibit some depth of 

 colour, and the larger veins, where a great number 

 of corpuscles are flowing together, will appear of 

 a decided red. To illustrate this by an example 

 familiar to the microscopist, a dozen glass slides laid 

 on each other, or a single slide viewed edgewise, will 

 appear green, although any of these when laid flat 

 and looked through singly, will, if made of good glass, 

 appear nearly colourless. 



The black markings and the star-like spots on the 

 fin are collections of pigment. Similar spots are 

 scattered over the web of the frog's foot on both 

 sides, but scarcely interfere with the view of the cir- 

 culation, because the lively movement of the latter 

 contrasts well with their stillness. But, as this con- 

 trast is wanting in a drawing, I have in the figures 

 omitted these spots, to allow the course of the circu- 

 lation to be traced without interruption. 



The curious observer who will catch a full-grown 

 frog, and like to hold it when caught, may observe a 

 few little pink blood-vessels crossing the web. The 

 foot being tightly held, they will probably appear 

 quite as evident as in fig. 3 ; and the reason of this 

 will be that they are veins, and that the grasping 

 of froggy's ankle has impeded their circulation. If 

 the frog, reptile-like, shows signs of preserving an 

 apathetic stillness, the grasp may be relaxed, and 



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