CHAPTER YII 

 THE VASCULAR SYSTEM 



THIS system includes the heart, arteries, capillaries, veins, and 

 lymphatic vessels. These structures form a continuous set of 

 branching tubes, which convey the blood from the heart, through 

 the arteries and capillaries, and back again through the veins to 

 the heart. In the capillaries a portion of the blood plasma trans- 

 udes into the tissue spaces, where it forms the " tissue juices," and 

 from which it is returned to the blood by the lymphatic vessels, 

 the terminal branches of which empty into the subclavian veins. 



This entire vascular system is completely lined by a single layer 

 of flattened epithelial cells, endothelimn, which are united edge to 

 edge by an intercellular cement substance, to form a continuous 

 membrane throughout the entire system. The blood vessels include 

 the arteries, capillaries, and veins, and these, together with the 

 heart, will form the subject of the present chapter ; the lymphatic 

 vessels will be described in connection with the lymphatic system. 



ARTERIES. The arteries convey the blood from the heart to 

 all the tissues of the body. They are therefore almost universally 

 present, but vary in size from the aorta down to minute unnamed 

 vessels of microscopic caliber. They are divisible, according to 

 size, into the large, medium sized, and small arteries, the arterioles, 

 and what may be termed the arterial capillaries, or " precapillary 

 arteries." The large arteries include only the aorta and the largest 

 of its immediate branches ; the medium-sized arteries comprise 

 nearly all the remaining named arteries of the body ; small arter- 

 ies, arterioles, and precapillary arteries include those unnamed 

 arteries which are to be found in nearly all of the organs and 

 tissues of the body. 



A medium-sized artery will be first described, as presenting the 

 typical arterial structure. Such a vessel consists of three coats : 



1. The internal coat tunica intima. 



2. The middle coat tunica media. 



3. The external coat tunica adventitia. 



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