884 THE VASCULAR SYSTEM. 
origin as far as the point at which the right internal mammary artery arises, is formed 
by the fourth right aortic arch; on the left side the fourth arch takes no part in the 
formation of the subclavian artery. 
In the lower limbs the primary main arterial stem, on each side, is represented by the 
sciatic, the popliteal, and the peroneal arteries. The sciatic artery arises from the caudal 
arch, and it, together with its prolongation through the popliteal space and leg, is probably 
formed from a somatic segmental vessel, but to which parts of this it corresponds is not 
clear. The external iliac artery, prolonged into the limb as the femoral artery, is 
developed at a later period than the sciatic artery. It arises from the caudal aortic arch 
above the origin of the sciatic artery, and, like the latter trunk, is probably a modified 
somatic segmental vessel. The femoral artery soon after its formation unites with the 
primary main stem, at the upper part of the popliteal space; the sciatic artery then 
atrophies and loses its connexion with the popliteal artery, and ultimately a permanent 
chief stem vessel is formed, which includes the external iliac, the femoral, the popliteal, 
and the peroneal arteries, and obviously it represents the two somatic segmental vessels 
by which it is formed. Both in the upper and in the lower limb, branches which attain 
a large size are given off from the main stem artery a short distance beyond the joint 
between the upper and middle sections of the limb, ¢.e. below the elbow in the upper limb 
and below the knee in the lower limb, and on account of the relatively great enlargement 
of these branches the continuity of the original stem is obscured. Thus it is that, in the 
adult, the brachial artery, the direct con- 
f \ tinuation of the stem which is divided into 
WA Lateral sinus sybelavian, axillary, and brachial sections, 
appears to terminate by dividing into the 
Primitive jugular ~_ radial and ulnar arteries, whilst originally 
it was continued through what, in the adult, 
Lie tewaleaet awali is the upper part of the ulnar artery to the 
anterior interosseous artery; the posterior 
interosseous, the radial, and the ulnar arteries 
a SS Subelavian being merely branches from the main stem. 
pou Similarly, in the lower extremity the 
popliteal artery, which is the continuation 
of the original stem artery, appears to ter- 
minate in the adult by dividing into the 
anterior and posterior tibial arteries, both of 
which in reality are branches from the sides 
of the main stem which was continued as 
the peroneal artery to the foot. 
Primitive jugular 
vein 
Duct of Cuvier 
Cardinal vein 
Vitelline vein 
Umbilical vein 
DEVELOPMENT OF THE VEINS. 
Simultaneously with the formation of 
the arteries by which the blood is distributed 
to the embryo and to the rest of the ovum, 
Renal vein and in a similar manner, a series of vessels 
is developed by means of which the blood is 
returned to the heart. These vessels are 
the veins, of which there are two main 
eroups. One group returns blood from the 
abdominal viscera and the annexa (the yolk- 
sac and allantois) ; the other group includes 
the vessels which return blood from the 
\ Wolffian bodies, the body wall, the head and 
: External iliac vein neck, and the limbs of the embryo. The 
Ret Lyn ee. first group consists of the vitelline, allantoic, 
Cardinal vein (internal ‘S A Soe 
| iliac) and portal veins ; the second group includes 
Fria. 603.—DEVELOPMENT OF THE VENOUS SYSTEM bee jugular and thescandinatiweiss 
(Diagrammatic). and the ducts of Cuvier. 
Stage I.—The ducts of Cuvier, the vitelline veins, and The trunks of the vitelline veins ang 
the umbilical veins open directly into the heart. formed by the posterior parts of the anterior 
; primitive ventral aortee, and necessarily they 
open into the posterior part of the heart (sinus venosus). They collect blood from the 
yolk-sac, and ascend along the vitello-intestinal duct to be continued upwards along the 
