CHAPTER VII 

 THE BLOOD VASCULAE SYSTEM 



This system includes the heart, arteries, capillaries, and veins. 

 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 transudes into the tissue spaces, where it forms the tissue 

 juices, and from which it is returned to the blood by the lymphatic ves- 

 sels, 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, the endothelium. The cells are united edge to 

 edge by an intercellular cement substance, to form a continuous mem- 

 brane throughout the entire system. The blood-vessels include the ar- 

 teries, capillaries, and veins, and these, together with the heart, will 

 form the subject of the present chapter. The lymphatic vessels (lymph 

 vascular system) will be described in connection with the lymphatic sys- 

 tem. The blood and lymph vessels together with their contents comprise 

 the vascular tissue. 



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 (innominate, 

 common carotids, subclavians and common iliacs), and the pulmonary 

 artery, the conducting arteries; the medium sized (distributing) ar- 

 teries comprise nearly all the remaining named arteries of the body; 

 small arteries, arterioles, and precapillary arteries include those un- 



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