32 ELEMENTARY PHYSIOLOGY 



tubes, with thicker walla, which are the smallest arteries, 

 on the one side and veins on the other, and these again 

 join on to larger arteries and veins, which ultimately com- 

 municate by a few principal arterial and venous trunks 

 with the heart. 



The mere fact that the walls of these vessels are thicker 

 than those of the capillai-ies constitutes an important 

 difference between the capillaries and the small arteries 

 and veins ; for the walls of the latter are thus rendered 

 far less permealile to fluids, and that thorough irrigation 

 of the tissues, which is effected by the capillaries, cannot 

 be performed by them. 



The most important difference between these vessels 

 and the capillaries, however, lies in the circumstance that 

 their walls are not only thickei", but also more complex, 

 being composed of several coats, one, at least, of which 

 is muscular. The number, arrangement, and even nature 

 of these coats differ according to the size of the vessels, 

 and are not the same in the veins as in the arteries, though 

 the smallest veins and arteries tend to resemble each 

 other. 



(i) The Structure of an Artery. — If we take one of the 

 smallest arteries, we find, first, a very delicate lining of cells 

 constituting a sort of epithelium continuous with the cells 

 which form the entire thickness of the wall of the capillaries. 

 Outside this comes the nuiscular coat, consisting of a thin 

 layer of muscle-fibres of the kind called plain or non- 

 striated (see Lesson VII.), made uj) of flattened spindle- 

 shaped cells with an elongated nucleus, wrapped round 

 the vessel at right angles to its length. Outside this 

 muscular coat is a thin layer of fibrous connective tissue 

 intermixed with a variable amount of fibres of elastic tissue 

 (see Lesson XII.). The walls of the smallest arteries 

 are thus seen to be made up essentially of three layers ; 

 the inner cellular, the middle muscular, the outer of connec- 

 tive tissue. The larger arteries are similarly composed of 

 three layers or coats, which are. however, thicker and 



