BLOOD-VASCULAR SYSTEM 29 1 



arteries on either side, an internal carotid artery which goes to the 

 brain, and an external carotid which supplies the more superficial 

 parts of the head. As was indicated above, the arteries arising from 

 the dorsal aorta are either somatic or visceral. Among the first 

 category are the intercostal and the renal arteries, the intercostals 

 coursing between the myotomes, the renals being distributed to the 

 successive segments of the excretory organs. Among these somatic 

 arteries an anterior and a posterior pair usually obtain especial promi- 

 nence, the first (the subclavian artery) supplying the fore limb, 

 while the iliac artery has similar relations to the hind limb. The 

 splanchnic or visceral arteries show little trace of metamerism. They 

 are distributed to the walls of the alimentary tract. Two pairs of 

 these vessels, again, are of special importance, a pair of omphalo- 

 mesenteric arteries in front, and a pair of hypogastric arteries near 

 the origin of the iliac arteries. 



The distal ends of these and other arteries are connected by 

 capillaries with the veins, through which the blood is returned to 

 the heart. The head is drained by a pair of (superior) jugular veins 

 above the mouth, while (in fishes) the region of the lower jaw and the 

 lower side of the gill arches have a pair of inferior jugulars. These 

 run back to the level of the hinder part (venous sinus) of the heart, 

 where they are joined on either side by a postcardinal vein coming 

 from the excretory organ. Jugulars and postcardinal of a side unite 

 to form a transverse trunk, the Cuvierian duct, which empties into 

 the sinus venosus. 



A pair of omphalo-mesenteric veins enter the posterior side of 

 the venous sinus. These pass on either side of the liver, and are 

 the continuations of a subintestinal vein which runs on the ventral 

 side of the alimentary canal. Behind, the subintestinal forms a loop 

 around the anus, beyond which it extends to the end of the tail as 

 the caudal vein. Subclavian veins from the fore limbs empty either 

 into the jugulars or the postcardinals near the Cuvierian duct, 

 while the blood leaves the hind limb of either side by an iliac vein, 

 which runs forward in the lateral body wall as the lateral abdominal 

 vein, to enter the Cuvierian duct. Of these veins, the omphalo- 

 mesenteric and subintestinal belong to the visceral group, the others 

 are somatic. 



Most of the vessels mentioned in the foregoing paragraphs are 

 laid down very early in the embryo, and the later developments are 

 largely concerned with modifications and additions to this frame- 



