208 



OF THE BLOOD AND CIRCULATION. 



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.* 



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

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



I 



i 



''//in, 



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

 escnlenta) 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 temporarid], 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. 



