352 THE ANIMAL KINGDOM. 



of which constitute the hypogastric arteries at a later period. The aorta 

 ends in a caudal artery. 



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, (i) 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 geni- 

 talia. 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 WolrHan 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 



