CHAPTER XXIV 



THE VASCULAR SYSTEM 



The vascular system is remarkably uniform in its main 

 features in all chordates. It consists essentially of four 

 longitudinal vessels running along the whole length of the 

 animal. Of these, one runs under the gut in the gut-wall 

 (subintestinal vessel) ; the other three run in the body-wall, 

 and are the dorsal aorta and the paired cardinal veins 

 respectively. The subintestinal vessel connects with the dorsal 

 aorta at the anterior end of the animal by a number of paired 

 vessels which run up round the gut on each side passing in 

 between the gill-slits. The anterior prolongations of the dorsal 

 aorta (which is paired in the anterior region) are the internal 

 carotids. Farther back the dorsal aorta gives off small vessels 

 in each septum (between the segments) to the tissues of the 

 body-wall, and other vessels which pass down the mesentery 

 supporting the gut to supply the gut- wall. The blood in the 

 gut-wall is collected up into the subintestinal vessel and is 

 led forwards again. On the way, it breaks up into capillaries 

 again in a glandular diverticulum of the gut — the liver, and 

 deposits much of the digested and absorbed material which it 

 has picked up in the posterior region of the gut (intestine). 

 In this way a hepatic portal system is formed. The blood in 

 the body- wall makes its way to the cardinal veins, and from 

 them it crosses the coelomic cavity between the body-wall and 

 the gut- wall by the ductus Cuvieri (or superior vena cava), 

 running in the transverse septum, to the subintestinal vein. 

 This is the fundamental type on which the peripheral vessels 

 are arranged in all chordates, and the details in the various 

 groups can be considered under the headings, veins, heart, 



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