152 



THE BLOOD AND ITS CIRCULATION. 



(smooth muscle, contractile cells). The first of these ele- 

 ments, elastic tissue, is found, with slight 

 exceptions only, at the summit of the 

 arterial cone, the aorta being formed 

 almost entirely of yellow elastic tissue. 

 On the other hand, the muscular element 

 predominates largely at the base of the 

 cone ; that is, in the coats of the small 

 arteries which precede the capillaries. In 

 the intermediate parts the elastic and mus- 

 cular tissues both share in the composi- 

 tion of the middle tunic, in proportion 

 to the distance at which the point under 

 consideration lies between the base and 

 summit of the cone ; so that a diagonal 



Fig. 49. Artery in line, dividing obliquely the thickness of 

 which the three coats the walls of the arterial cone, represents 



are dissected. ,1 _.i . i_ i 



exactly the comparative richness in elas- 

 tic and muscular tissue of the different parts of the arterial 

 walls (Fig. 50). 



Fig 50. Arterial cone ; composition of 

 the arterial coats.* 



Fig. 51. Natural form of the 

 arteries, f 



The arteries are, owing to the presence of the muscular 

 and yellow tissue, extremely elastic tubes. This fact alone 



* Proportion of 'the elastic and the muscular element in the composition of 

 the coat of the cone from summit (At) to base (c, c). 1, Muscular element. 

 2, Elastic element. 



f Element by which the natural form of the arteries is determined. A, Ap- 

 pearance of the section of an artery, supposing it to be formed of muscular 

 tissue only. B, Section of an artery supposed to be of elastic tissue only. 

 C, I), Section of an artery, showing 'its actual ribbon-like form, which is tne 

 physiological result of the struggle between these two elements, the elastic and 

 the muscular. 



