The primitive condition of the arterial system is retained in Pisces, but in Amphibia, and especially in Amniota, changes take place, more particularly affecting the aortic arches. The third aortic arch (i.e. ist branchial arch of Fish) forms the root of the internal carotid, and the stem of the common carotid, the external carotid being continued forwards from its base. The fourth arch, i. e. 2nd branchial, becomes the aorta, the left persisting in Mammalia, the right in Aves, both left and right in Reptilia and Amphibia. The fifth arch ( = 3rd branchial) becomes the pulmonary artery, the left in Mammalia forming both arteries, the right both in Ophidia - the left and right persisting in other Amniota. The pulmonary artery of Amphibia, however, is developed as a rule from a sixth arch (i. e. the 4th branchial), the fifth arch (3rd branchial) either persisting as a rudiment or disappearing altogether (see account of the class). The artery to the lung in Dipnoi, or to the swimming bladder in some Ganoidei, is derived also from the sixth arch, or 4th branchial.
As to the venous system. A subintestinal vein continued backwards from the heart is formed in all embryo Fish. Its posterior section, present also in higher forms, constitutes the caudal vein which unites with the cardinal veins (infra) and loses its connection with the praecaudal portion. The praecaudal portion persists in the typhlosole of Petromyzon and the spiral valve of Elasmobranchii. It is lost in other Vertebrata, but gives off during development, (1) the vitelline veins which subsequently become the roots of the portal vein, and also (2) the portal network of vessels as well as that of the hepatic veins in the liver. The part of the vessel (ductus venosus) between the origins of the two networks eventually undergoes atrophy. To the portal system are subsequently connected visceral veins, and sometimes veins from the abdominal wall (= epigastric) and the genitalia. In Pisces a right and left venous trunk - the ductus Cuvieri - open into and really form the sinus venosus. The outer ends of these ducts are continued forwards to the head as the anterior, backwards (and dorsal to the Wolffian body) as the posterior, cardinal veins. The former represent the external jugulars to which the subclavian veins (veins of fore-limbs) become connected.
The posterior cardinals become united in Pisces with the caudal vein and resolved in part into the renal-portal venous system. In other Vertebrata they disappear except at their anterior and posterior extremities - the ductus Cuvieri then forming the venae cavae superiores which receive the jugular and subclavian veins. A vena cava inferior is developed in Amphibia and higher Vertebrata. It is connected anteriorly with the sinus venosus or, where this is absent, with the right auricle. In Amphibia and Reptilia it receives the efferent renal veins and the hepatic veins; in Aves and Mammalia the veins of the hind-limb as well; and in the latter the caudal vein. A renal-portal circulation or supply of venous blood to the kidneys exists in all Amphibia, in Reptilia except Chelonia, and possibly in Aves, in addition to the arterial supply from the aorta. The blood from the hind-limbs passes in Amphibia partly through the renal-portal system, and partly through an epigastric vein, as it does in Crocodilia, through the renal-portal alone in Lacertilia, through an epigastric alone in Chelonia. The epigastric vein lying in the ventral wall of the abdomen exists in Amphibia and Reptilia, and in the embryoes of Aves and Mammalia where it is lost in the adult.
Primitively double, it rarely remains so (Chelonia, Crocodilia), but becomes single by the atrophy of one of the two veins. Its anterior end is connected with the hepatic portal system. Its posterior extremity is variously connected to the renal-portal system and hind-limbs (supra), but in the embryoes of Sauropsida and Mammalia to the allantois, a foetal membrane, which is developed as a ventral outgrowth of the posterior extremity of the mesenteron. And it may be noted that veins from the allantoid bladder, the homologue of the allantois, fall into the epigastric in Amphibia.
The arteries and veins are connected peripherally by the capillary vessels which form networks in the various tissues, some few, e.g. epidermis, epithelium, cartilage, etc. excepted. The capillaries have a simple epithelial wall, the smaller arteries and veins add a coat of transverse, i. e. circularly disposed non-striated muscle cells, to which the larger vessels superadd coats of connective tissue. The larger vessels are formed apparently from solid cords of cells, the central cells being set free and becoming blood corpuscles, the peripheral being converted into the walls of the vessels. Capillaries are formed in many instances by the development of a vacuole in a branched cell, the branches and vacuoles of adjoining cells becoming connected. Certain of the veins in higher Vertebrata possess valves which ensure a flow of blood in a constant direction. The blood consists of a plasma containing in suspension 'white' corpuscles or leucocytes which possess the power of amoeboid motion. The red haemoglobin-containing corpuscles or haematids are oval and nucleated in all Vertebrata except Mammalia, where they are circular and non-nucleated. They have a fixed outline.
The leucocytes are formed for the most part in the lymphatic system (infra).
A spleen - a structure with a lacunar blood-vascular circulation - is found in all Vertebrata in connection with the mesogastrium. It possesses a blood-making function. There is also a lymphatic system of spaces and vessels formed in the mesoblastic tissues. The lymphatic system communicates with the coelome, and with the blood-vessels at certain spots either anteriorly (Mammalia) or anteriorly and posteriorly. Contractile lymph hearts are sometimes found close to the points of communication except in Mammalia. The lymphatic channels of the intestine and mesentery constitute the absorbent or lacteal system. The larger vessels have well defined and independent walls, and in Aves and Mammalia valves like the veins. A large lymphatic vessel or reservoir, the ductus thoracicus, lies at the back of the thorax in Aves and Mammalia. It communicates with the two venae cavae superiores in the former, with the left one only in Mammalia. There is a lymph sinus in the same position in Amphibia and Reptilia. This sinus, like the duct, receives lymph from the hinder extremities, body walls and viscera.