CHAPTEE XII 



THE VASCULAR SYSTEM, THE LYMPHATICS, AND THE 

 BLOOD-GLANDS 



The Cyclostomata and Pishes 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 supplied 

 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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