20 ELEMENTARY BIOLOGY. [CHAP. 



A. Arteries. Efferent in relation to the heart, breaking up 

 into lesser branches. 



i. The system of the anterior aortic arch (carotid 

 trunks), distributing blood to the parts of the head 

 generally and to the brain. 



2. The system of the middle aortic arch (aortic trunk), 

 distributing blood to the body and limbs, together 

 with the organs of the viscera other than the lungs. 



3. The system of the posterior aortic arch (pulmo- 

 cutaneous trunks), distributing blood to the lungs 

 and skin. 



B. Veins. Afferent in relation to the heart, formed by the 

 union of lesser factors (they break up only in the case 

 of the portal veins). 



1. The system of the superior cava, formed on each 

 side by the union of the veins bringing back the 

 blood from the head, fore-limbs, and parts adjacent. 

 Special veins (great cutaneous) are concerned in the 

 return of the blood from the skin. 



2. The system of the inferior cava, formed by the union 

 of the efferent-renal, genital and hepatic veins. 



3. The system of the anterior abdominal and renal-portal 

 veins; formed by the bifurcation of the veins coming 

 in from the hind-limbs. The former receives blood 

 from the urinary bladder and the body-wall, and 

 at its anterior end divides into two branches 

 which go to the corresponding lobes of the liver. 

 The latter gives off branches (afferent renal veins) 

 which break up in the substance of the kidney. 



4. The system of the vena porta (hepatic-portal vein) 

 formed by the union of the veins which bring back 



