326 HISTORY OF THE HUMAN BODY 



laries, in which the exchange of respiratory gases takes place, 

 and when it is re-collected into the efferent branchial arteries, 

 corresponding in number to the afferent branchials, the blood 

 has become aerated. These latter arteries converge to the 

 median line, where they unite to form a median aorta, which 

 lies upon the ventral side of the vertebral centra, and gives 

 off the main arteries of the body. Before the arches of the 

 two sides unite they give off the carotid arteries, which supply 

 the head and brain ; and then, not far from the point of union, 

 the subclavians, to the anterior paired limbs (pectoral fins). 

 Lower down appear branches that supply the body walls and 

 the viscera; and the posterior paired limbs (ventral fins) are 

 supplied by the iliacs. As these branches are given off, the 

 aorta diminishes in size and terminates at the end of the tail 

 as a mere thread, protected throughout the caudal region by 

 the haemal arches of the vertebrae. 



The entire body is thus supplied with aerated blood from 

 a single main channel with its branches, but on its return its 

 course is not so simple, and involves three distinct venous 

 systems connected with one another by capillaries. The first 

 of these consists of four great longitudinal veins, the two 

 anterior and the two posterior cardinals, which collect the 

 blood from the head, the anterior fins, and the walls of the 

 trunk. As in the embryological sketch, the anterior and pos- 

 terior cardinal veins of each side unite into a ductus Cuvieri, 

 which enters the sinus venosus. Associated with the posterior 

 cardinals are the two large lateral veins which lie in the body 

 wall and were perhaps originally situated along the bases of 

 the lateral fin-folds, from which the paired limbs have been 

 derived. They arise as very small vessels along the sides of 

 the tail and enlarge rapidly as they proceed anteriorly through 

 the assumption of tributary branches from each somite. In the 

 cloacal region the two lateral veins communicate by numerous 

 anastomosing branches, forming a cloacal plexus (represented 

 in the diagram by a single vein), and receive the iliac veins 

 from the posterior fins. Anterior to this they still receive meta- 

 meric contributions from each somite and finally empty 



