i : MORPHOLOGY OF THE VEINS. 895 
apparently the latter, for as the upper section of the inferior vena cava grows downwards from 
the right hepatic vein it divides into two branches, right and left, each of which joins the 
corresponding cardinal vein close to the termination of the renal vein. Before it fuses with the 
cardinal vein of its own side, each of the terminal branches of the upper section of the inferior 
vena cava gives off branches which ultimately become the suprarenal and spermatic veins ; there 
can be no doubt, however, that both the suprarenal and spermatic veins are intermediate visceral 
segmental veins, and in all probability they originally terminated in the corresponding cardinal 
veins, their development from the upper section of the inferior vena cava being due to the 
production in the embryo of a condition which has been secondarily acquired during the 
development of the species. If this is the case, it is extremely probable that the left spermatic 
vein represents not only an elongated intermediate visceral segmental vein, but also a portion of 
the left cardinal into which it opened. 
The vena azygos major is the persistent upper portion of the left cardinal vein, a fact 
which is emphasised by its frequent connexion with the inferior vena cava at the level of the 
right renal vein. The right intercostal veins which open into the vena azygos major are somatic 
segmental veins, the upper three or four of which have united together by pre-costal anastomoses 
to form a right superior intercostal vein. 
The vertical portions of the left azygos veins are remnants of the left primitive cardinal 
vein, and their transverse portions are enlarged transverse anastomoses comparable to the left 
innominate and left common iliac veins. The left, like the right intercostal veins, are segmental 
somatic veins ; but whilst the right superior intercostal vein is formed by pre-costal anastomoses 
between the upper three or four dorsal somatic segmental veins, the left superior intercostal vein 
(Fig. 606) represents the upper part of the left cardinal vein and the part of the left primitive 
jugular vein below the transverse anastomosis, which becomes the left innominate vein ; 
moreover, the left superior intercostal vein frequently retains in the adult a connexion with the 
oblique vein of Marshall, which represents the left duct of Cuvier, by means of which both the 
lett primitive jugular and the left cardinal veins originally communicated with the heart. 
Visceral Veins.—The portal vein represents portions of the ventral longitudinal anastomosing 
vessels, being derived from the vitelline veins. 
The pyloric vein is a splanchnic intersegmental ventral longitudinal anastomosing vein. The 
coronary vein is partly a ventral and partly a dorsal splanchnic intersegmental longitudinal 
anastomosis, and the superior and inferior mesenteric veins are dorsal splanchnic longitudinal 
intersegmental venous anastomoses, the splenic vein being merely a tributary from a lymphoid 
organ developed in the dorsal mesentery. 
The facial vein is a combination of somatic and splanchnic veins of several segments, and the 
internal maxillary vein is probably of similar nature. The thyroid and bronchial veins return 
blood from organs developed from diverticula from the walls of the alimentary canal; they are, 
therefore, more or less modified segmental splanchnic veins ; so also apparently are the vesical and 
the middle and inferior heemorrhoidal veins. 
The cardiac veins are simply “vasa vasorum,” and they belong therefore to the splanchnic 
group of vessels, but it is impossible to say whether they are segmental or intersegmental. The 
coronary sinus into which they open is a portion of the sinus venosus of the heart, and therefore 
of an originally intersegmental vessel. 
The hepatic and pulmonary veins are new vessels which return blood to the heart after the 
liver and lungs have been interposed in the vascular system. 
It is noteworthy that some parts of the splanchnic venous system, 7.e. the portal vein and the 
coronary sinus, are portions of the most primitive vascular system, and that others, ze. the 
thyroid, bronchial, mesenteric, vesical, and hemorrhoidal veins, appear to belong to a somewhat 
secondary group of splanchnic veins of combined segmental and intersegmental character ; more- 
over, some of the secondary group of veins open into the primary splanchnic veins, e.g. the 
superior and inferior mesenteric into the portal vein; some open into the dorsal longitudinal 
anastomosing veins, e.g. the vesical and hemorrhoidal veins open into the cardinal veins, which 
are intersegmental anastomoses ; others again open into an entirely new vein, viz. the internal 
jugular, which is developed along the dorso-lateral border of the fore-gut, and which is therefore 
comparable to the dorsal venous splanchnic intersegmental anastomosis, which in the abdominal 
region becomes converted, after fusion of the vessels of opposite sides, into the greater parts of the 
superior and inferior mesenteric veins; the latter, however, open into a ventral longitudinal 
anastomosing vessel, the portal vein, whilst the former joins a dorsal longitudinal anastomosing 
trunk. 
Veins of the Limbs.—The veins of the limbs, like the arteries, were probably at one time 
segmental in character, but we have no absolute proof that this was the case. Looked at from 
an embryological standpoint, the most primitive limb veins are a superficial distal arch and 
a post-axial trunk vein in each extremity ; at a later period digital veins are connected with the 
distal arch, and a pre-axial trunk is formed. In the upper extremity the distal arch and its 
tributaries remain as the dorsal venous arch and the digital veins, and the post-axial vein becomes 
the posterior ulnar, the basilic and axillary veins, and also that part of the subclavian vein which 
hes external to the termination of the external jugular vein, the remainder of the subclavian 
vein being formed from the primitive jugular vein itself. The pre-axial vein of the upper 
extremity is represented in the adult by the radial and cephalic veins ; the latter vessel originally 
terminated in the external jugular vein above the clavicle, the union with the axillary portion of 
the post-axial vessel being a secondary condition ; the primary condition is, however, frequently 
retained in man, and is constant In many monkeys. The anastomosis between the pre-axial 
