208 



OF THE BLOOD AKD CIBCULATION. 



vessels which appertain to it, give rise to the greatest diversity 

 of form in the peripheral vascular system, which has never- 

 theless so determinate a character in each tissue, that an ex- 

 amination with the microscope of the smallest particle of a 

 finely injected preparation enables us to say with certainty 

 from what part of the body it was obtained.* 



[ 3/3. When a transparent part of a cold-blooded animal, 

 the web of the frog's foot, for example, is examined under a 



Fig. 225. Membrane between two of the toes of the frog's (Rana 

 esculenta] hind-foot, with the vessels and their anastomoses, drawn under 

 the lens, and magnified three diameters, a c, Veins, b b, Arteries. 



way, and are excellent subjects, but they require more delicate handling 

 than tadpoles. The circulation in the allantois of the young embryos of 

 lizards and snakes is also a very beautiful sight, when these subjects can 

 be had at the proper point of evolution ; they require to be removed from 

 the ova, and observed covered with fluid albumen in a watch-glass. In 

 the winter, frogs are the best subjects ; fishes are then much less proper. 

 In the web of the hind foot of the common frog (Rana tempor aria), the 

 circulation is perhaps seen to as great advantage as anywhere. All our 

 better microscopes are now provided with a stage adapted for placing the 

 animal, which is best secured by being put into a linen or calico bag, with 

 tapes at each corner to tie it down. 



* Professor Wagner's Physiology, p. 286. 



