32 PHYSIOLOGY FOR BEGINNERS CHAP. 



The circulation of the blood can be easily seen in the web 

 of the frog's foot. The frog is a most useful animal for studying 

 physiology. Its value depends chiefly on its being what is 

 called a " cold-blooded " animal, that is, an animal whose 

 temperature varies with the temperature around it, and on 

 the fact that its tissues live for a long time after the brain is 

 destroyed and sensibility lost, and after the animal as a whole 

 is dead. 



Cut off the head of a frog as low down as possible with a 

 sharp pair of scissors. It is thus made quite incapable of 

 feeling or movement. Cut a hole half an inch across in a thin 

 piece of wood, and pin out the web between two of the toes 

 of the frog over the hole (it will be better if a ring of cork is 

 glued round the hole to put the pins into), and examine the 

 most transparent part of the web with the low power of the 

 microscope. 



We have seen in the dissection of the rabbit the large 

 artery, the aorta, which carries the blood from the heart to the 

 organs and the large veins, the superior vena cava and the 

 inferior vena cava, which carry it back to the heart again. 

 The aorta gives off branches to the several parts of the body, 

 and these branches divide again into other branches, and 

 so the branching can be traced on to a large number 

 of small arteries in the various organs. In a similar way, 

 the large veins can be seen to be made up by the union 

 of a large number of small veins coming from the various 

 organs. The blood passes from the small arteries to the small 

 veins of an organ in very minute tubes the capillaries. 

 Capillaries, with the blood passing along in them, can be seen 

 easily in the web of the frog's foot. The web consists of thin 

 skin on each side, with a little connective tissue between the 

 two layers of skin. In this tissue blood vessels lie. If the 

 web is held up to the light, small vessels will be seen as fine 

 red lines branching away from the larger vessels running along 

 at the side of each toe ; with the microscope, a network of 

 branching vessels will be seen throughout the web. The 

 finest of these are the capillaries, the larger are the small 

 arteries bringing the blood from the heart, and the small veins 

 carrying it back to the heart. 



Blood-vessels are in all cases not mere spaces in the tissue, 



