322 



THE BLOOD AND ITS CIRCULATION 



the right auricle through two large vessels known as the vence 

 cavcB. It requires only from twenty to thirty seconds for the 

 blood to make the complete circulation from the ventricle back 

 again to the starting point. This means that the entire volume of 

 blood in the human body passes three or four thousand times a 

 day through the various organs of the body.^ 



Portal Circulation. — Some of the blood, on its way back to the heart, 

 passes to the walls of the food tube and to its glands. From there it is sent 

 with its load of absorbed food to the liver. Here the vein which carries 

 the blood (called the portal vein) breaks up into capillaries around the 

 cells of the Uver, when it gives up sugar to be stored as glycogen. From 

 the liver, blood passes directly to the right auricle. The portal circula- 

 tion, as it is called, is the only part of the circulation where the blood 

 passes through two sets of capillaries on its way from auricle to auricle. 



Circulation in the Web of a Frog's Foot. — If the web of the foot 

 of a live frog or the tail of a tadpole is examined under the com- 



■d 



Capillary circulation in the web of a frog's foot, as seen under the compound micro- 

 scope, a, b, small veins ; c, pigment cells in the skin ; d, capillaries in which 

 the oval corpuscles are seen to follow one another in single series. 



1 See Hough and Sedgwick, The Human Mechanism, page 136. 



i 



