CHAPTER X 



STRUCTURE AND PROPERTIES OF BLOOD-VESSELS- 

 REGULATION OF THE CIRCULATION LYMPHATICS 



Structure of Blood-vessels 



THE internal layer of the blood-vessels, a membrane con- 

 tinuous with the endocardium lining the interior of the heart, 

 is called the endothelium. It is continuous throughout the 

 arteries, capillaries, and veins. The endothelium consists of 

 flat, very thin cells, united together at their edges to form a 

 thin membrane. 



Capillaries. The smallest vessels, the capillaries, which 

 vary a good deal in width, and may be only ^^th of an inch 

 wide, have this thin membrane for their wall, and nothing else. 

 A capillary, in fact, consists of thin, flat cells, united together 

 at their edges to form a tube. These cells are longer than 

 they are broad ; in each cell lies a flattened nucleus. From 

 this simple structure there is a gradual transition with 

 increasing thickness of the wall to the large arteries on the 

 one side and to the large veins on the other. 



Arteries. The smallest arteries have their walls 

 strengthened by the presence of a few plain, muscular fibres 

 placed round outside the endothelium. The plain muscular 

 fibres are small spindle-shaped cells, each with its long 

 nucleus, and are wrapped round the endothelium across the 

 length of the vessel. As we trace these smallest arteries back 

 until they join to form arteries larger than themselves, but still 

 so small that a microscope is required to see them, we find that 

 the muscle fibres have increased in number, till they form a 



