DISEASES OF THE HEART AND BLOOD-VESSELS. 81 



The arteries divide and subdivide (like the branches of a tree) be- 

 come smaller and smaller, and ultimately ramify into every part of the 

 body, terminating in a network of very small tubes called capillaries, 

 which can only be recognized by the aid of a microscope. The walls 

 of the capillaries differ considerably from the walls of the arteries and 

 veins, and they also vary according to the size of the capillary tubes; 

 they are so thin and peculiar that white corpuscles and the fluid part 

 of the blood readily pass through them into adjacent tissue. The capil- 

 laries terminate in veins. 



The veins take the blood from the capillaries in all parts of the body. 

 They begin in very small tubes which unite to become larger in size 

 and less in number as they approach the heart. The veins, like the 

 arteries, have three coats, but are' thinner, less muscular, and less 

 elastic than the arteries, and collapse when empty. 



In its course an artery is iisually accompanied by a vein, and in many 

 in stances by two veins; in the latter case it is placed between them, 

 and if but one vein accompanies it, the artery is always the deeper and 

 least exposed of the two vessels. Nerves also generally accompany 

 the arteries. Muscles in many locations serve to point out the situation 

 of arteries, as they often are in contact with them in their course; cer- 

 tain arteries are in contact witli bones; and in certain situations they 

 lie in the connective tissue between the skin and bone, which is the 

 case with some of these vessels about the head and legs ; such arteries 

 are best for ascertaining the state of the pulse. Arteries deeply seated 

 in certain places are in contact with bones, and the knowledge of their 

 location enables the surgeon to compress them against the bone for the 

 purpose of arresting the flow of blood in a particular vessel in case of 

 hemorrhage or during operations. The more important arteries are 

 placed deep within the body; but in those cases where they are super- 

 i;ci;il, tlu-y are generally found where least exposed to injury, as for 

 example, on the inner side of the legs. Arteries are less numerous 

 than veins, and the total capacity of the arteries is much less than that 

 iie veins. A great number of veins are in the tissue immediately 

 beneath the skin, and these are not generally accompanied by arteries. 

 All the superficial veins, and many of the others, have valves within 

 t hem to prevent the reflux of blood; the faces of the valves are towards 

 the heart, and when anything interferes to retard the flow of blood to 

 tin- heart, the valves are closed by the backing of the blood, and effect- 

 ually prevent its return. By the same means these valves favor the 

 llo\\- of blood to the heart, because of the pressure assured by the con- 

 lion of muscles adjacent to the veins. There are no valves in th; 

 vein*, of the lungH. 



The. blood throughout its course, in the heart, arteries, capillaries, and 

 veins, is inclosed within these vessels. There is no opening into the 

 course of the blood, except where the large- lymphatics empty into the 

 v< nous blood. 



