Footnote 46: See Hough and Sedgwick, _The Human Mechanism_, page 136. 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 liver, when it gives up sugar to be stored as glycogen. From the liver, blood passes directly to the right auricle. The _portal circulation_, 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. [Illustration: Capillary circulation in the web of a frog's foot, as seen under the compound microscope. _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.] 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 compound microscope, a network of blood vessels will be seen. In some of the larger vessels the corpuscles are moving rapidly and in spurts; these are _arteries_. The arteries lead into smaller vessels hardly greater in diameter than the width of a single corpuscle. This network of _capillaries_ may be followed into larger _veins_ in which the blood moves regularly. This illustrates the condition in any tissue of man where the arteries break up into capillaries, and these in turn unite to form veins. Structure of the Arteries.--A distinct difference in structure exists between the arteries and the veins in the human body. The arteries, because of the greater strain received from the blood which is pumped from the heart, have thicker muscular walls, and in addition are very elastic. Cause of the Pulse.--The _pulse_, which can easily be detected by pressing the large artery in the wrist or the small one in front of and above the external ear, is caused by the gushing of blood through the arteries after each pulsation of the heart. As the large arteries pass away from the heart, the diameter of each individual artery becomes smaller. At the very end of their course, these arteries are so small as to be almost microscopic in size and are very numerous. There are so many that if they were placed together, side by side, their united diameter would be much greater than the diameter of the large artery (_aorta_) which passes blood from the left side of the heart. This fact is of very great importance, for the force of the blood as it gushes through the arteries becomes very much less when it reaches the smaller vessels. This gushing movement is quite lost when the capillaries are reached, first, because there is so much more space for the blood to fill, and second, because there is considerable friction caused by the very tiny diameter of the capillaries.