THE PIGEON 169 



the liver to the right limb of the loop of the duodenum. The 

 shorter duct is also large, and enters the left limb not far from 

 the gizzard. 



Observe the shape of the pancreas, which lies in the duodenal 

 loop. Find its three ducts : two of these leave the pancreas near 

 its middle and enter the right limb of the loop ; the third duct 

 leaves the pancreas in front of the other two and passes forward 

 to the right limb of the loop. 



Exercise 6. Draw the dorsal aspect of the liver, showing the branch- 

 ing of the portal vein, the two bile ducts, the duodenum, and the 

 pancreas and its ducts. 



The Vascular System. This is made up of the following 

 organs: (1) the heart, (2) the arteries, (3) the veins, (4) the 

 capillaries, and (5) the lymph vessels. The heart is composed of 

 two auricles and two ventricles. Two systems of arteries are 

 present: (a) the pulmonary arteries, which take venous blood 

 from the right ventricle to the lungs to be aerated; and (b) the 

 systemic arteries, which take arterial blood from the left ven- 

 tricle to the various tissues and organs of the body. Two systems 

 of veins are present: (a) the pulmonary veins, which bring arterial 

 blood from the lungs to the left auricle; and (b) the systemic 

 veins, which bring venous blood from the tissues and organs of 

 the body to the right auricle. 



The systemic veins may be further subdivided into two sys- 

 tems: these are {a) the caval veins, those veins carrying blood 

 directly to the heart, and their branches; and (b) the portal sys- 

 tems of veins, which carry blood from certain tissues and organs 

 directly to the liver and kidneys, from which it is afterward 

 taken to the heart by the postcaval vein. 



The Portal Veins. Two portal systems of veins are present, 

 the hepatic portal and the renal portal system. The renal portal 

 circulation is not, however, complete in birds, only a small part of 

 the blood brought by the renal veins being distributed through 

 capillaries to the substance of the kidneys, the greater part of it 

 passing directly to the iliac veins and thence to the postcaval vein 

 and the heart. 



