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 differs according to 

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



arteries it is chiefly composed of elastic 

 fibres, with a few unstriped muscular fibres 

 interspersed amongst them. In the smaller 

 arteries the elastic tissue becomes progres- 

 sively less and less marked as they diminish 

 in size, being replaced by the muscular 

 tissue, which at last forms almost the whole 

 thickness of the middle coat, the fibres for 

 the most part running in a circular direc- 

 tion. The internal coat is composed of a 

 sheet of elastic tissue with large apertures 

 in it. It is lined by a layer of flat, endo- 

 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 

 wall. The several coats of the arteries 

 endow them with strength to enable them 



^ j t th pressure O f the blood, and also 

 r ' 



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



Muscular Fibres, e, Tunica Adventitia, show- . . . . . > i i i i 



ing connective-tissue Fibres and Corpuscles, 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 facility. But on the 

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

 immediately recoil. The first effect 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 



Fig. 192. Transverse Section through a 

 small Artery and Vein 



A, Artery, v, Vein, a, Endothelial Cells 

 with Nuclei. b, Elastic Layer of Tunica 



