180 HANDBOOK OF PHYSIOLOGY. 



like a watch-pocket (7, fig. 148). In the centre of the free edge of the 

 pouch, which contains a fine cord of fibrous tissue, is a small fibrous 

 nodule, the corpus Arantii, and from this and from the attached border 

 fine fibres extend into every part of the mid substance of the valve, 

 except a small lunated space just within the free edge, on each side of 

 the corpus Arantii. Here the valve is thinnest, and composed of little 

 more than the endocardium. Thus constructed and attached, the three 

 semilunar pouches are placed side by side around the arterial orifice of 

 each ventricle, which can be separated by the blood passing out of the 

 ventricle, but which immediately afterward are pressed together, so as 

 to prevent any return (6, fig. 147, and 7, fig. 148). This will be again 

 referred to. Opposite each of the semilunar cusps, both in the aorta 

 and pulmonary artery, there is a bulging outward of the wall of the 

 vessel: these bulgings are called the sinuses of Valsalva. 



Structure. The valves of the heart are formed essentially of thick 

 layers of closely woven connective and elastic tissue, over which, on 

 every part, is reflected the endocardium. 



The Arteries. 



Distribution. The arterial system begins at the left ventricle in a 

 single large trunk, the aorta, which almost immediately after its origin 

 gives off in the thorax three large branches for the supply of the head, 

 neck, and upper extremities; it then traverses the thorax and abdomen, 

 giving off branches, some large and some small, for the supply of the 

 various organs and tissues it passes on its way. In the abdomen it 

 divides into two chief branches, for the supply of the lower extremities. 

 The arterial branches wherever given off divide and subdivide, until the 

 calibre of each subdivision becomes very minute, and these minute ves- 

 sels pass into capillaries. Arteries are, as a rule, placed in situations 

 protected from pressure and other dangers, and are, with few exceptions, 

 straight in their course, and frequently communicate (anastomose or 

 inosculate) with other arteries. The branches are usually given off at 

 an acute angle, and the areas of the branches of an artery generally ex- 

 ceed that of the parent trunk, and as the distance from the origin is 

 increased, the area of the combined branches is increased also. After 

 death, arteries are usually found dilated (not collapsed as the veins are) 

 and empty, and it was to this fact that their name (aprypia, the wind- 

 pipe) was given them, as the ancients believed that they conveyed air 

 to the various parts of the body. As regards the arterial system of the 

 lungs, the pulmonary artery is distributed much as the arteries belong- 

 ing to the general systemic circulation. 



Structure. -The walls of the arteries are composed of three principal 

 coats, termed (a) the external or tunica adventitia, (b) the middle or 

 tunica media, and (c) the internal or tunica intima. 



