238 HUMAN PHYSIOLOGY 



departure from the type. The blood which is supplied to 

 the alimentary canal (below the esophagus) is not re- 

 turned directly to the great vein which passes up through 

 the abdominal cavity on its way to the right auricle. 

 This blood, together with that from the pancreas and the 

 spleen, is gathered into a vessel called the portal vein. 

 This is unique among blood-vessels of any size, inasmuch 

 as it is made by the union of small branches like any 

 vein while it is destined to subdivide again and to 

 supply a set of capillaries as though it were an artery. 

 The second set of capillaries is in the liver. 



The blood which has passed through the tissue of the 

 liver is collected by a number of short veins which empty 

 into the inferior vena cava. This circulatory apparatus 

 of the liver is spoken of as the portal system and it will be 

 seen that its chief peculiarity consists in the fact that the 

 capillaries are " tandem" to those of the digestive organs, 

 receiving what may be called second-hand blood. The 

 obvious suggestion is that the liver is interposed between 

 the alimentary organs and the rest of the body to work 

 over the absorbed food principles before admitting them 

 to the circulation at large. The liver receives a secondary 

 supply of arterial blood by a relatively direct channel 

 from the aorta. 



The Pulmonary Circulation. It has been said that 

 the right ventricle has a lighter duty than the left. It 

 has to pump the same quantity of blood in a unit of time 

 but the mass which it has to keep moving is much 

 smaller. The pressure which it is required to develop is 

 not more than a third as great as that which the left 

 ventricle must apply. The blood is sent up from the 

 right ventricle through a short, wide pulmonary artery 

 (carrying venous blood). Under the arch of the aorta 

 the pulmonary artery forks to supply the two lungs. 

 The blood comes back to the left auricle arterialized and is 

 brought in by four pulmonary veins, two from each side. 



The Lymph and its Movement. The term lymph is 

 usually given to all the fluid outside the blood-vessels. 



