ARTERIES, VEINS, AND CAPILLARIES 



187 



better protected. The necessity for this is found in the 

 fact that the bleeding of a cut artery with its pulsing blood 

 is much more difficult to stop than a bleeding vein. 



If we prick any spot on the skin with a needle we get 

 bleeding, even though no arteries or veins are visible. 

 This is due to the fact that a network of tiny tubes, too 

 small to see with the eye, and called hair tubes or capilla- 

 A 



FiGr. 70 — Capillary circulation in tlie web of a frog's foot (X 100) ; a, &, small 

 veins, d, capillaries in which the corpuscles are seen to follow one another in 

 single series ; c, pigment cells in the skin. 



lies, because of their tiny dimensions, connect veins and 

 arteries. The relation of arteries, veins, and capillaries 

 may be observed to good advantage by stretching the 

 living web of a frog's foot or tadpole's tail under the 

 objective of a compound microscope. (See Ex. XLV. and 

 Fig. 70.) The blood vessels, which are transparent, show 



