66 The Vertebrate Organ Systems 



spleen, intestine, and cloaca ; the coeliac supplies the stomach, liver, and 

 pancreas. Posterior to the coeliaco-mesenteric artery, the dorsal aorta 

 gives off several small arteries which supply the kidneys and gonads. 

 Finally the dorsal aorta divides into the two common iliacs, which con- 

 tinue into the legs as the sciatic arteries. 



The head region is supplied by branches from the carotid arches, 

 or as they are often called, the common carotids. This blood vessel 

 divides into the external and internal carotids. Branches from the ex- 

 ternal carotid supply the tongue and neighboring muscles, while branches 

 from the internal supply the brain, the eyes, and the roof of the 

 mouth. At the point where the branching of the common carotid occurs 

 is a swelling, the carotid gland, which helps maintain an even blood 

 flow. 



The Capillaries. — The arteries branch and gradually become 

 smaller, finally terminating in the tiny, thin-walled blood vessels known 

 as the capillaries. The walls of these blood vessels are so very thin 

 that materials in the blood can easily diffuse out to the body cells. In 

 turn, waste materials can diffuse into the blood. Capillaries occur in 

 tremendous numbers throughout all parts of the body. As these start 

 converging, larger vessels are formed which ultimately become the veins. 



The Venous System. — The veins are the blood vessels which carry 

 the blood back toward the heart. Usually their walls are much thinner 

 than those of the arteries and they are provided with valves for the 

 prevention of the backward fiow of the blood. The blood carried by 

 the veins is usually unoxygenated. In the frog, there are two excep- 

 tions to this generalization : the pulmonary vein returning blood from 

 the lungs to the left auricle, and the cutaneous vein returning blood 

 from the skin to the sinus venosus. 



The venous system of the frog is more complex than is the arterial 

 system. This is due to the fact that there are two portal systems: 

 the renal portal and the hepatic portal. A portal system is a series 

 of blood vessels beginning and ending in capillaries. 



All the blood from the systemic circulation returns to the sinus 

 venosus by means of three principal veins : the right and left anterior 

 venae cavae and the posterior vena cava. Each anterior vena cava is 

 formed of three branches : the external jugular, the innominate, and 

 the subclavian. The external jugular brings the blood from the floor 

 of the mouth and tongue region ; the innominate from the shoulder 

 muscles, the brain, roof of the mouth, and some head muscles; the 



