894 THE VASCULAR SYSTEM. 
or ovarian veins are possibly developed as offsets; moreover, it must not be forgotten that the 
veins of the extremities are, like the extremities themselves, secondary structures, and that they 
are developed at a later period than the veins of the trunk, with which, however, they ultimately 
communicate. 
In the light of these facts the morphology of the chief veins. of the trunk and limbs may 
now be considered. 
The external jugular vein is obviously a portion of an intersegmental anastomosis, for it is part 
of the primitive Jugular vein, which originally extended from the internal occipital protuberance 
to the post-condyloid foramen, and thence to the duct of Cuvier. During the course of develop- 
ment the intracranial part of the primitive jugular, on each side, is converted into the horizontal 
portion of the lateral sinus and the occasionally persistent squamo-petrosal sinus ; outside the 
cranium the trunk of the temporo-maxillary vein, the whole of the external jugular vein, and 
that portion of the subclavian vein which intervenes between the external and internal jugular 
veins, are formed from the primitive jugular. On the right side the right innominate vein 
and the upper part of the superior vena cava are also formed from the primitive jugular vein, 
whilst on the left side the lower portion of the vessel becomes the upper part of the left superior 
intercostal vein. 
After the formation of the limbs the primitive jugular receives the pre-axial and post-axial 
veins of the fore-limb of the same side, which pass respectively along the radial and ulnar 
borders of the limb; both join the primitive jugular vein, the former above and the latter 
below the clavicle. Subsequently, however, the pre-axial vein of the fore-limb loses its connexion 
with the primitive jugular vein, and opens below the clavicle into the post-axial vein, and the 
upper part of that vessel becomes the outer part of the subclavian vein, 7.z. that portion of the sub- 
clavian vein which extends from the outer border of the first rib to the entrance of the external 
jugular vein, the remainder of the subclavian being formed by the portion of the primitive 
jugular vein which intervenes between the entrance of the pre-axial vein and the junction with 
the internal jugular vein. 
The internal jugular vein is a newly formed anastomosing vessel which commences from the 
primitive jugular vein at the root of the neck and grows upwards to the base of the skull, 
where it passes through the jugular foramen, and ascends along the inner surface of the mastoid 
portion of the temporal bone to join the lateral sinus, of which it becomes the sigmoid portion. 
It probably represents a dorsal splanchnic intersegmental venous anastomosis. 
The innominate vein of the left side is an enlarged transverse anastomosis between the two 
primitive jugular veins, and the corresponding vessel on the right side is the portion of the 
right primitive jugular vein which lies between the origin of the right internal jugular vein and 
the transverse anastomosis between the two primitive jugular veins. 
The superior vena cava is also formed from the primitive longitudinal anastomosis on the 
right side; the upper portion, which lies above the entrance of the azygos vein, being the lower 
part of the right primitive jugular vein, and the lower portion, which is enclosed within the peri- 
cardium, is the persistent right duct of Cuvier. 
The only other vein formed from the jugular portion of the dorsal longitudinal anastomosis 
is the upper part of the left superior intercostal vein, which represents the part of the left 
primitive jugular vein lying below the transverse anastomosis which becomes the left innomi- 
nate vein; occasionally this part of the left primitive jugular vein becomes enlarged, and forms 
a vertical left innominate vein which terminates in a left superior vena cava, the latter being 
formed from the left duct of Cuvier. This arrangement is the regular and normal condition in 
many mammals. 
The internal iliac veins, the right common ilac vein, the lower part of the inferior vena 
cava, the vena azygos major, and the vertical portions of the upper and lower left azygos veins, 
and part of the left superior intercostal vein, are all parts of the primitive cardinal veins. They 
represent, therefore, portions of the dorsal longitudinal intersegmental anastomoses. 
The internal iliac veins are the persistent lower sections of the cardinal veins, and their 
visceral and parietal tributaries probably represent more or less modified splanchnic and somatic 
segmental veins. 
The left common iliae vein may, in thé lower part of its extent, represent the part of the left 
cardinal vein immediately above the junction of the pre-axial hind-limb vein with the latter 
vessel, but the greater part of it is an enlarged transverse anastomosis between the cardinal veins 
at the level of the pelvic brim. The right common iliac vein, on the other hand, is the portion 
of the right cardinal vein which hes between the entrance of the limb vein and the transverse 
anastomosis which becomes the left common iliac vein. The inferior vena cava, from its com- 
mencement to the entrance of the renal veins, is a portion of the right cardinal vein, and the 
right lumbar veins which terminate in it are the somatic segmental veins of the right side of the 
lumbar region ; whilst the left lumbar tributaries are the left lumbar segmental veins, which have 
been transmitted across the middle line by transverse anastomosing channels which connected 
the lumbar sections of the cardinal veins together. 
The upper part of the inferior vena cava is a new anastomosing channel formed between the 
upper end of the right hepatic vein and the right cardinal vein. This section of the inferior 
vena cava grows downwards from the right hepatic vein. * 
The right and left renal veins originally terminated in the corresponding cardinal veins, 
and are therefore probably intermediate visceral segmental veins, but that part of the left renal 
vein which crosses the middle line is either an enlarged transverse anastomosis between the 
cardinal veins, or an outgrowth from the lower end of the upper part of the inferior vena cava ; 
