CHAPTER XV 



VASCULAR SYSTEM 



The blood-vascular system is the distributing mechanism of the 

 body. Food materials are absorbed by the epithelium of the diges- 

 tive tract, and oxygen is diffused through the lining of the lungs, 

 and these pass into the blood. In the more complex animals 

 the means of distribution are very important, for substances 

 necessary for metabolism cannot be absorbed directly by the 

 individual cells of the organism. Therefore an efficient system 

 of distribution has been vital in the evolution of the higher 

 groups. 



The chordates, and particularly the vertebrates, have a closed 

 system. The epithelial lining of the vessels is continuous. The 

 arteries divide to form capillaries, and the arterial capillaries 

 become venous capillaries without any break. The blood, there- 

 fore, courses through unbroken channels. A certain amount of 

 the vascular fluids seep through the capillary walls and sur- 

 round the cells. These fluids are collected by the lymph vessels 

 (page 243) which empty into, and have a lining continuous with, 

 the veins. 



The typical chordate system is found in Amphioxus. The 

 ventral, pulsating artery [ventral aorta) functions as a heart 

 and forces the blood forward along the ventral vessel lying 

 under the pharynx. Paired branchial arteries pass from the aorta 

 to the gills on either side, where each breaks into capillaries. The 

 capillaries from each gill collect to form an efferent branchial 

 artery, and these in turn form two dorsal aortae, one on either 

 side of the notochord. At the posterior end of the pharynx these 

 fuse to form a single median aorta lying ventral to the notochord. 

 Arteries are given off to the muscles and viscera, supplying them 

 with food and oxygen, taking up the waste products of metabo- 



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