xiii.] THE FROG. 209 



a V-shaped notch about the size of a spread-out 

 frog's web : place the frog on the board, belly down- 

 wards, and fix it by passing round it two or three 

 turns of tape : next tie threads round the toes of one 

 hind-foot, and by means of them spread out the web 

 over the notch in the board, taking great care that it 

 is only very lightly stretched. The animal should be 

 kept moist by a bit- of wet blotting-paper spread over 

 its back. 



2. Examine the web with i inch obj. : Note 



a. The black pigment-cells in the skin ; sometimes 

 irregularly branched ; sometimes more compact. 



b. The close network of blood-vessels lying deeper 

 than the pigment-cell layer. 



a. The arteries, running mainly towards the free 

 edge of the web, and constantly diminishing 

 in size as they give off branches ; the blood- 

 flow in them from larger to smaller branches. 



/?. The capillaries, in which the arterial branches 

 end : small vessels forming a close network 

 and frequently branching or anastomosing 

 without much altering their size. 



y. The veins, formed by the ultimate union of 

 the capillaries, and increasing in size by union 

 with one another; the blood-flow in them 

 from smaller to larger trunks. 



c. The blood-flow: the current being marked by 

 the solid bodies (corpuscles) carried along in the 

 fluid : it is most rapid in the arteries ; slowest, 

 and most uniform, in the capillaries. 



M. 14 



