THE VEINS. 825 
dorsal digital branch which passes beneath the tendon of the extensor hallucis to the 
inner side of the great toe. 
(5) The princeps hallucis (plantar digital artery) springs from the termination of 
_ the dorsalis pedis in the sole of the foot; it runs forwards in the plantar part of the 
_ first imterosseous space, and divides at the interdigital cleft into collateral digital 
branches for the supply of the adjacent sides of the first and second toes on their plantar 
aspects. Before its division it supplies a plantar digital branch to the inner side of 
the great toe. 
THE VEINS. 
Veins commence at the terminations of the capillaries. They converge towards 
the heart, and unite with one another to form larger and still larger vessels, until 
finally seven large trunks are formed which open into the auricles of the heart. 
Three of these, the superior vena cava, the inferior vena cava, and the coronary sinus, 
belong to the systemic circulation ; they contain venous blood, and open into ine 
right auricle. The remaining four belong to the pulmonar y circulation ; they return 
arterialised blood from the lungs, and open into the left auricle. 
In addition to the systemic and pulmonary veins, there is also a third group of 
veins, constituting the portal system, in which blood from the abdominal part of 
the alimentary canal, and from the spleen and pancreas, is conveyed to the liver. 
The portal system is further peculiar in that it both begins and ends in capillaries. 
From its terminal capillaries in the liver the hepatic veins arise, and as these open 
into the inferior vena cava the blood of the portal system is finally poured into the 
general systemic circulation. The hepatic veins also receive blood supplied to the 
liver by the hepatic arteries. 
PULMONARY VEINS. 
The terminal pulmonary veins (v. pulmonales, Figs. 552 and 545), two on each side, 
open into the left auricle of the heart. Their tributaries arise in ¢ apillary plexuses 
in the walls of the pulmonary alveoli. By the union of the smaller veins larger vessels 
are formed which run along the anterior aspects'of the bronchial tubes and, uniting 
together, ultimately form a single efferent vessel in each lobe, which passes into the 
root of the lung. Thus there are five main pulmonary veins, but, immediately 
after entering the root of the lung, the vessels from the upper and middle lobes of 
the right lung join together, and so only four terminal pulmonary veins open into 
the left auricle of the heart. Neither the main stems nor their tributaries possess 
valves. 
Relations.—In the root of the lung the upper pulmonary vein on each side hes 
below and in front of the pulmonary artery. The lower pulmonary vein on each 
side is in the lowest part of the root, and it is placed much farther back than the 
upper vein. 
On the right side the upper pulmonary vein passes behind the superior vena cava, and 
the lower behind the right auricle. They both terminate in the upper and back part of 
the left auricle close to the interauricular septum. 
On the left side both upper and lower pulmonary veins cross the front of the descend- 
ing aorta, and they terminate in the upper and back part of the left auricle near its left 
border. 
All four pulmonary veins perforate the fibrous layer of the pericardium, and receive 
partial coverings of the serous layer before they enter the auricle. 
SYSTEMIC VEINS. 
The systemic veins return blood to the right auricle of the heart through the 
superior vena cava, the inferior vena cava, and the coronary sinus. The two first- 
named receive blood from the veins of the body and limbs and from most of the 
a : 
