CIRCULATORY SYSTEM 775 



In all vertebrates the heart lies on the ventral side of the digestive 

 tract covered by a pericardial sac. This sac is really a part of the 

 coelomic lining. The various large blood vessels carrying blood from 

 the heart to the general system are known as aortae, and the large veins 

 returning the blood directly into the heart are usually called venae cavae. 



The ventral aorta gives off various pairs of vessels called the aortic 

 arches, which are situated on each side of the pharynx in the grooves 

 called the gill septa. These arches run from the ventral aorta around 

 the digestive canal to the dorsal side where they unite to form a longi- 

 tudinal canal. That is, the arches along each side form a separate canal 

 at first, then the two canals unite to form the dorsal aorta, which runs 

 caudad the entire length of the body. There may be, and usually are, 

 various small arteries arising from any or all of these arterial arches. 

 It is necessary that the student know what becomes of the aortic arches 

 and in what groups of animals certain ones disappear and others remain 

 functional. The first pair of arches lying toward the head end give rise 

 to both the internal carotid artery which goes to the brain, and the ex- 

 ternal carotid supplying the more superficial portion of the head. The 

 arteries which arise from the dorsal aorta are either somatic or splanch- 

 nic, that is, either supply outer or internal portions of the body. Exam- 

 ples of somatic blood vessels are the intercostal (intersegmental) arteries 

 running between the ribs, while the mesenteric arteries, which are dis- 

 tributed primarily to the alimentary canal, are of the splanchnic type. 



The subclavian artery, which supplies the arms of the animal, and 

 the iliac artery, which supplies the hind limbs, are some of the larger and 

 more common of the somatic arteries. 



The splanchnic or visceral arteries do not show much trace of seg- 

 mentation. They are distributed to the walls of the digestive tract. Two 

 pairs, however, of these vessels are of special importance, namely, a pair 

 of omphalomesenteric arteries in front, and a pair of hypogastric 

 arteries (internal iliac) near the origin of the iliac arteries (Fig. 450). 



There are really no end arteries or veins. All arteries carry blood to* 

 certain parts of the body through minute capillaries which then anasta- 

 mose with the venous capillaries which drain the various parts which 

 the arteries supply. 



The head is drained by a pair of jugular veins which are found above 

 the mouth. In fishes there are also a pair of inferior jugulars in the 

 region of the lower jaw and the lower side of the gill arches. These run; 

 caudad to the level of the sinus venosus, where they are joined by a post 

 cardinal coming from the excretory organs. The jugular and post cardi- 

 nal on each side unite to form a trunk which rims transversely and 

 empties into the sinus venosus. This is called the Cuvierian duct. A 

 pair of omphalomesenteric veins enters the sinus venosus from the caudal 

 side. These are continuations of a subintestinal vein running* alongside 

 of the liver after having passed along the ventral side of the digestive 



