CIRCULATION AND THE BLOOD SYSTEM 73 



Examine the skin on the back of the hand between two 

 veins. Can you see any blood vessels? Place the finger 

 on this part. Can you feel any pulse? Prick through the 

 skin at this point with a sterilized needle. Does the punc- 

 ture bleed by spurts or steadily? The small blood vessels 

 filling these places are called capillaries on account of their 

 small size (capillus=a, hair). They connect the veins and 

 arteries. 



XLV. CIRCULATION IN A FROG'S FOOT. 



Apparatus. Compound microscope, cover slip, live frog, shingle, 

 wet absorbent cotton, and cloth. 



Directions. Bind a live frog in wet absorbent cotton, 

 leaving one leg extended. Fasten the frog, so bound in place, 

 on a frog board (a piece of shingle with a hole the size of a 

 cover slip at one end). Stretch the web of the foot over the 

 hole in the board. Fasten it securely, with the stretched 

 web as level as possible. Mount this board on the micro- 

 scope stage in such a way as to bring the web-covered hole 

 under the objective of the microscope. With a pipette 

 place a drop of water on the top of the web, and cover with 

 a piece of cover slip. Illuminate in the usual way and focus 

 first with the low and then with the high power. 



Note the network of blood vessels and the slow-moving 

 stream of corpuscles within them. Are the corpuscles the 

 same size and shape as those in the human blood? Is there 

 more than one kind? Observe that in some of the blood 

 vessels the blood moves in spurts at regular intervals. What 

 kind of vessels are these? Does the blood in these flow from 

 or toward the body? Follow the course of the blood from' 

 these into the smaller tubes where the corpuscles move in 



