STRUCTURE OF ARTERIES. 



'45 



of Kolliker, which differ in no essential respect from the 

 fibres of organic muscle, such as those which compose the 

 muscular coat of the stomach and intestines. To this part 



Fig. 41.* 



of the middle coat the name of 

 muscular was applied by Hunter. 



These two elements of the 

 middle coat exist in different rela- 

 tive amounts in different arteries ; 

 and, in general, are in an in- 

 verse ratio to each other, for 

 the arteries which possess most 

 elastic tissue have least mus- 

 cular tissue, while those whose 

 walls are most muscular, are in 

 general least elastic. In the 

 large arteries, such as the aorta 

 and its main branches, scarcely 

 a trace of the muscular element 

 can be found, nearly the whole thickness of their walls 

 consisting of elastic tissue. But in the arteries farther 

 removed from the heart, and of smaller size, the pro- 

 portionate thickness of the elastic element gradually 

 diminishes, while, as a general rule, that of the mus- 

 cular element progressively increases. Moreover, in the 

 arteries of certain organs, probably of those in which the 

 supply of blood is subject to greater than usual variations, 

 in adaptation to fluctuations in the amount of function they 

 discharge, there is proportionately greater development of 

 the muscular tissue. 



Of the properties which the arteries possess in these 

 two tissues, the muscularity has its seat of course exclu- 

 sively in the muscular tissue, and no artery without this 

 element would present any contraction similar to that of 



* Fig. 41. Muscular fibre- cells from human arteries, magnified 350 

 diameters (Kolliker). a, natural state; b, treated with acetic acid. 

 L 



