1OI2 VASCULAR SVSTEM 
combined trunk being then known as the v. cava inferior. But the 
left v. hepatica magna receives also the v. wmbilicalis, a long unpaired 
vein arising in adult birds from the walls of the abdomen and its 
air-sacs, and frequently anastomosing with the epigastric veins. Its 
Cente of he stem runs nearly midway along 
vena hepatica the visceral surface of the abdo- 
dextraand sinistra minal wall, passing first to the 
v.hep. sinistra right of the stomach and then 
:.LeftLiverlobe between the two lobes of the 
Mess liver, finally joining the left hep- 
atic vein. ‘This peculiar vessel is 
the remnant of one which played 
an important part in the embryo, 
for it originally collected all the 
blood of the yolk-sac (p. 211) 
into one stem which passed along 
the left side of the gut; and, 
after receiving the mesenteric 
vein, entered the right auricle 
of the heart as the v. omphalo- 
mesaraica. "This stem, however, 
soon became surrounded by the 
liver, and began to form the 
4, hypogastr. Hepatic Portal System by partly 
stnistra breaking up into capillaries, while 
the mesenteric developed more 
and more, until the primitive ves- 
sel persists only as the umbilical 
DIAGRAM OF A Birp’s PorTAL VENOUS vein. 
System. Ventral Aspect. 
yebtlical 
vein 
= 
v.port. |v 
dext, 
v.mesent. 
(W.cocerg. 
Owing to the numerous anas- 
tomoses set up by veins on the confines of their several districts, 
some of these connexions are often used by the blood as “short 
cuts,” and then become wider channels, while the original vessels 
suffer atrophy, so that quite new modifications are brought about. 
Such variations are so common, especially in the cervical and 
pelvic regions, that they deprive the Venous System of much of 
its taxonomic value.! 
B. Lymphatic System. 
The Lacteal or Absorbent vessels arise in the villi of the 
intestine, whence they convey the chyle into the Lymphatics 
(p. 139), together with the white blood or lymph corpuscles 
(p. 43), which are produced in the follicles at the base of the villi 
1 A careful, minute and amply illustrated description of the venous system 
of numerous birds—‘‘ Systema venosum Avium cum eo Mammalium et inprimis 
Hominis collatum’’—was published by Neugebauer in 1844 (Nov. Act. L.-C. 
Acad. xxi. pp. 517-698, tabb. xxxvi.-1.). 
