296 COMPARATIVE ANATOMY 



are formed as forward continuations of the collecting vessels 

 into which the first three arterial arches open on the dorsal 

 side. The middle parts of the mandibular and hyoid arches 

 disappear as shown in the diagram. The first branchial arches 

 persist to form the proximal parts of the carotid arteries, but 

 their dorsal connections with the second branchial arches are 

 lost. The second branchial arch of the left side becomes the 

 aortic arch curving round to the left to pass into the dorsal 

 aorta. The arch of the. right side loses all connection with 

 the dorsal aorta, but its proximal portion persists to form the 

 subclavian artery. The third branchial arches disappear al- 

 together, and so does the fourth arch of the right side. The 

 pulmonary artery is formed as an outgrowth of the proximal 

 side of the left fourth arch, and the remainder of the arch 

 is aborted or persists as a solid ligamentous cord passing 

 from the pulmonary artery to the aorta, forming the so-called 

 ductus arteriosus of human anatomy. In connection with 

 these changes, and the division of the primitively single 

 ventricle into right and left halves, the proximal part of the 

 truncus arteriosus is divided into two. The right division 

 retains its connection with the right ventricle, and becomes 

 the base of the pulmonary artery, the left division forms the 

 base of the aorta. 



In the venous system of mammals, the most characteristic 

 thing is the entire absence of a renal-portal system. The 

 inferior vena cava reaches to the hind end of the abdominal 

 cavity and is formed by the union of the external and internal 

 iliac veins bringing back blood from the hind limbs, with 

 the ilio-lumbar veins bringing back blood from the walls of the 

 posterior part of the trunk. The blood from the kidneys 

 is returned to the inferior vena cava by a pair of renal veins, 

 and the kidneys are supplied with blood only by the renal 

 arteries. In the mammalian embryo, the venous system 

 consists of a pair of anterior and a pair of posterior cardinal 

 veins, and is therefore fish-like. The anterior cardinals form 

 the superior venae cavae of the adult. The posterior cardinals 

 are replaced by the inferior vena cava, but the anterior part 

 of the right posterior cardinal persists as the azygos vein, and 

 in man the left posterior cardinal is represented by the hemi- 

 azygos, which discharges into the azygos by a transverse 

 connection. (The absence of a renal-portal system is not 



