442 HEALTH AND DISEASE 



it from the heart to all parts of the body. Of large size where they 

 commence, the pulmonary artery from the right ventricle and the aorta 

 from the left ventricle, they divide and subdivide almost always at an 

 acute angle till they terminate in the capillaries. They possess three 

 tunics or coats. The outer one, sometimes named the adventitia, is thin, 

 strong, and resistant, and is composed of connective tissue, with some 

 elastic fibres; in it run the small vessels and nerves which supply the 

 walls of the vessels themselves. The middle coat diflers according to 

 whether a large or a small artery is under observation. In the larger. 



Q arteries it is chiefly composed of elastic 



fibres, with a few uustriped muscular fibres 

 i interspersed amongst them. In the smaller 



ll arteries the elastic tissue becomes progres- 



if sively less and less marked as they diminish 



I in size, being replaced by the muscular 



i»\-^--^^j.-i^^~^ tissue, which at last forms almost the whole 



.' ^'♦T *~^'' ^ thickness of the middle coat, the fibres for 



'^""//y ~ the most part running in a circular direc- 



I ' ^ \ tion. The internal coat is composed of a 



~* *, VVi sheet of elastic tissue with large apertures 



»i « A J in it. It is lined by a layer of flat, endo- 



""^ A( thelial cells, which are therefore in contact 



^ / with the current of blood traversing the 



vessels. The nerves of the arteries form 



^ ■ -.- . net-works in the substance of the vessel 



Fig. 192.— Transverse Section through a Wall. The Several coats of the artcrics 

 small ArteiT and Vein endow them wlth Strength to enable them 



A Artery. V, Vein a, Endothelial Cells ^q j.ggig^ ^J^g prCSSUrC of the blood, and alsO 

 with Nuclei. 6, Elastic Layer of Tunica ^ 



intima. c. Tunica Media, d. Nuclei of its with elasticity and contractiUty. The elas- 

 tlToZlMlTtislnlnZl^^M^^^ ticity is best marked in the large arteries, 



the contractility in the smaller ones. Both 

 properties fulfil very important purposes. With each beat of the heart 

 a pint or more of blood is suddenly injected into each of the great 

 arteries. The shock and jar that this would produce through the entire 

 system is almost entirely abolished by the great elasticity of the walls 

 of the pulmonary artery and aorta. These vessels yield, and, greatly 

 widening, receive the new column of blood with fixcility. But on the 

 instant of the heart ceasing to deliver the last drop of its contents, they 

 immediately recoil. The first efiect of the recoil is to close the semilunar 

 valves, the next to cause the blood to move onwards and distend the 

 next part, of the artery in front. This having expanded, though to a 



