888 THE VASCULAR SYSTEM. 
which crosses the aorta below the superior mesenteric artery, remains as the inner portion 
of the left renal vein, which therefore receives the left spermatic and left suprarenal veins 
as tributaries. 
THE VEINS OF THE LIMBS. 
Two sets of veins are developed in each limb, the superficial and the deep ; the former 
are the primary vessels, and as a rule they are quite apart from the limb arteries ; the 
deep veins are secondary, and they accompany the arteries of the limb. 
At the peripheral extremity of each limb a venous arch is developed, which is subse- 
quently transformed into the digital veins. In the upper extremity the arch terminates on 
the ulnar (post-axial) side of the limb in a trunk which afterwards becomes the posterior 
ulnar, basilic, axillary, and subclavian veins. At a later period additional superficial 
vessels are formed, and of these a median vein which drains the palm, and a radial which 
commences on the radial side of the dorsum of the hand, are the most important. The 
radial passes up the preaxial border of the limb, becomes the cephalic, and for a time 
terminates in the primitive jugular vein ; this connexion is usually lost,! and a new com- 
munication is formed with the axillary vein. The median vein ends in an anastomosing 
vessel between the basilic and cephalic veins at the elbow, and through which it also 
communicates with the deep veins. 
In the lower extremity the peripheral venous arch terminates in a fibular or post-axial 
trunk, which remains in the adult as the external saphenous vein ; its connexion with the 
sciatic vein, which was its original continuation upwards, is soon lost, and a new com- 
munication is formed with the popliteal vein. 
The internal saphenous vein is a later development which appears on the pre-axial 
border of the limb, and terminates in the femoral vein. 
The deep veins appear as a series of anastomosing channels at the sides of the - 
arteries. 
THE PULMONARY VEINS. 
The pulmonary veins develop simultaneously with the lungs, and at first the veins 
from both lungs unite to form a single trunk, which enters the left auricle posteriorly, 
close to the auricular septum ; subsequently the single trunk is absorbed, and two veins, 
one from each lung, enter the left auricle, and eventually, as the result of further 
absorption, two veins from each lung terminate in that cavity. 
MORPHOLOGY OF THE VASCULAR SYSTEM. 
In conformity with the general plan of the vertebrate body, the vascular system is essentially 
segmental in character. This is obvious, even in the adult, in the intercostal and lumbar vessels. 
It is distinguishable, though less obvious, in the vessels of the head and neck and of the pelvis. 
The segmental arteries and veins form a series of bilaterally symmetrical vessels, each of 
which is united to the vessels of adjacent segments by intersegmental channels, which anastomose 
with one another, through the portions of the segmental vessels which they connect together, 
and thus form longitudinal trunks. The longitudinal trunks are clearly secondary, and they 
are mainly, though not exclusively, intersegmental. From them the main stem vessels of the 
individual are formed, and from or to these latter the segmental vessels appear to proceed as 
branches or tributaries. . 
In the course of development the secondary character of the longitudinal trunks is lost sight 
of ; they become the most important trunks in the individual, and they are formed before the 
segmental vessels make their appearance. 
THE SEGMENTAL ARTERIES AND THEIR ANASTOMOSES. 
The main longitudinal trunks formed by the intersegmental vessels anastomosing with-each 
other, through the segmental arteries which they connect together, are the primitive aorte. The 
descending aorta is formed, in the greater part of its extent, by the fusion of the dorsal parts of 
the primitive aorte, and from it the segmental arteries arise in pairs. 
In a typical segment of the body of the embryo there are three segmental arteries on each 
side. One rises from the dorsal surface of the primitive dorsal aorta, ze. from the dorsal longi- 
tudinal trunk, and runs outwards in the tissues developed from the somatic mesoderm ; it is 
distributed to the body wall, including the spinal column and its contents, and is termed a 
somatic segmental artery. A second vessel rises from the side of the primitive dorsal aorta ; it 
is distributed to the structures developed from the intermediate cell mass, viz. the suprarenal 
1 Tn certain cases it remains, and then the cephalic vein crosses the front of the clavicle and terminates 
in the external jugular vein. 
