CHAPTER XII 
THE VASCULAR SYSTEM, THE LYMPHATICS, AND THE 
BLOOD-GLANDS 
THE Cyclostomata and Fishes possess a closed vascular system, 
consisting of a heart, arteries, capillaries, and veins, the whole 
forming a continuous series of blood-containing channels provided 
with definite limiting walls, through which the blood is propelled 
in a constant direction by the rhythmical contractions of the 
heart. In the course of the circulation the blood flows from the 
heart through a single large trunk, the ventral aorta, to the capil- 
laries of the gills. From the gills the arterialised blood is collected 
into a large dorsally-situated vessel, the dorsal aorta, and thence 
is distributed through a system of arteries to the capillaries of 
the various organs of the body. Finally, the blood is collected 
from the capillaries and returned to the heart by the veins. 
Although in most instances the organs of the body are supphed 
with arterialised blood conveyed to them by arteries, there are 
nevertheless cases in which an organ may receive venous blood by 
a vein in addition to arterial blood supplied by an artery. For 
example, the capillaries of the liver not only receive blood from 
the hepatic artery, but also venous blood by a large vein (hepatic 
portal vein), formed by the union of a number of smaller veins 
by which venous blood is collected from the capillaries of the 
stomach, intestine, spleen, and pancreas. In this and similar 
instances, where a vein formed by the union of the capillaries of 
an organ, or of a series of organs, instead of uniting with other 
veins and proceeding towards the heart, becomes continuous with 
a second set of capillaries in some other organ, a “ portal” system 
is said to be formed, and in the particular example of the liver 
it is termed the “hepatic portal” system. <A similar, or “renal 
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