THE ARTERIES AND VEINS, 183 



The cutaneous artery (Fig. 80, au) is a branch of the fourth 

 efferent vessel, which arises very close to the pulmonary artery, 

 but independently of this, and at a rather later stage. 



5. The Veins. 



The veins, in the earlier stages of their development, are 

 chiefly characterised by their large size, and irregular lacunar 

 character. 



a. The vitelline veins are the first veins to be formed in the 

 body. They appear as irregular lacunas in the mesoblast of the 

 splanchnopleure, along the sides of the yolk-mass and of the liver 

 diverticulum, and unite in front to form the sinus venosus or 

 most posterior part of the heart ; they carry to the heart the 

 food matter absorbed from the yolk-mass. 



About, or shortly after, the time of hatching, the liver diver- 

 ticulum becomes more definitely bounded, and the vitelline and 

 hepatic veins become distinct from one another ; later still, by 

 folding of the wall of the hepatic diverticulum, the hepatic veins 

 are carried deeply into the substance of the liver (Figs. 64 and 

 76), as already described in the section dealing with the deve- 

 lopment of the liver (p. 154). 



b. The sinus venosus (Fig. 71, RS) is at first formed merely 

 by the union of the vitelline veins, but very early becomes a 

 definite transverse vessel, running across the body in close con- 

 tact with the anterior wall of the liver, and opening into the 

 auricular portion of the heart by a large anterior aperture. 



c. The anterior cardinal veins are paired, and return blood 

 from all parts of the head, except the floor of the mouth. Each 

 is formed by the union, behind the ear, of two principal veins : — 

 the jugular vein, which returns blood from the brain and dorsal 

 part of the head ; and the facial vein, which runs more super- 

 ficially along the side of the head, ventral to the eye and ear. 



d. The posterior cardinal veins (Fig. 84, vc) are also paired, 

 and are in special relation with the head kidneys, which they 

 completely surround. They are of enormous size during the 

 early stages of tadpole life, when the head kidneys are function- 

 ally active, forming vascular networks which occupy the spaces 

 between the tubules of the head kidneys. Each posterior car- 

 dinal vein receives somatic veins from the hinder part of the 

 body wall, and unites, in front, with the anterior cardinal 



