528 THE BLOOD-VASCULAR SYSTEM 



exceptions to dichotomous branching; branches may run perpendicularly or recurrently to the 

 vessel from which they arise; or several branches may arise simultaneously. 



There is less tendency to anastomosis between large or medium sized arteries than in veins 

 of corresponding magnitude. Anastomoses do occur, however, particularly in the form of 

 arches, such as the palpebral, plantar and volar arches, or the arches between the intestinal 

 arteries. This form of anastomosis is sometimes called inosculation. Between smaller arteries 

 anastomosis is usually free as in the case, for instance, of the articular retia. In some organs 

 anastomosis (excepting capillary) between neighbouring arteries can scarcely be said to exist 

 at all; the a. centrahs retinae affords a good example of this, as do the arteries of the brain, spleen, 

 and kidney; such arteries are called terminal. 



The larger arteries are supplied by vasa vasorum, frequently arising from their own recurrent 

 branches. 



The veins [venae] have thin walls, and after death are either collapsed or 

 filled with clot or stained serum. They are characterized by the presence of 

 valves and frequent anastomoses. 



Frequent anastomoses occur between veins of all sizes; plexuses are common, such, for 

 instance, those of the pelvis. Vence comitantes are veins which, usually in pairs, accompany 

 many arteries; they communicate with one another, around the artery, very freely. Veins 

 do not primitively accompany arteries. In the case of the extremities the primitive veins are 

 superficial. The deep veins of the limbs are of later formation and to them the superficial 

 veins subsequently become tributary. 



The veins from the stomach, spleen, pancreas and intestine are collected into a large trunk, 

 the portal vein. This does not open into the inferior vena cava directly, but breaks up into 

 numerous smaller vessels in the hver. From these the blood is returned, through the hepatic 

 veins, to the inferior cava. 



Many veins are provided with valves, the free borders of which are directed toward the heart. 

 In the small veins the valves are single; in the larger veins they are usually double, rarely treble. 

 Valves are much more numerous in the veins of infants than those of the adult, they seem to 

 disappear progressively with advancing age. The venous valves are most numerous in the 

 superficial veins, and in the deep veins of the extremities; in many veins of the head and neck 

 they occur only at their point of termination in a larger trunk. 



The cranial venous sinuses are modified veins, consisting of intima only which hues channels 

 in the fibrous dura mater. The venous spaces in cavernous tissue, such as the corpora cavernosa, 

 may be looked upon as specially modified veins. 



The larger veins, hke the arteries, have vasa vasorum. 



The arteries and veins will be considered in the following order: 1, pulmonary 

 artery and veins; 2, the systemic arteries; and, 3, the systemic veins. At the 

 ends of the second and third divisions, the development and variations are 

 considered. 



1. THE PULMONARY ARTERY AND VEINS 



The pulmonary artery [a. pulmonalis] (fig. 441) passes from the right ventricle 

 to the lungs. It differs from all other arteries in the body in that it contains 

 venous blood. It arises as a short, thick trunk from the conus arteriosus of 

 the right ventricle, and, after a course of about 5 cm. (2 in.) within the pericar- 

 dium, divides into a right and a left branch. These branches pass to the right 

 and the left lung respectively. 



The trunk of the pulmonary artery at its origin is on a plane anterior to the 

 ascending aorta, and slightly overlaps that vessel. Thence it passes upward, 

 backward, and to the left, forming a slight curve around the front and left side of 

 the ascending portion of the aorta; and, having reached the concavity of the 

 aortic arch, on a plane posterior to the ascending aorta, it divides into its right 

 and left l)ranches, which diverge from each other at an angle of about 130°. 

 The divi.sion of the pulmonary artery occurs immediately to the left of the second 

 left chondrostcrnal articulation. 



In the foetus, the pulmonary artery continues its course upward, backward, and to the left 

 under the name of the ductus arteriosus (Botalli), and opens into the descending aorta just 

 below the origin of the loft .sul)cluvian artery. After birth, that portion of the pulmonary 

 artery which extends to the iiorta becomes obliterated, and remains merely as a fibrous cord, 

 the ligamentum arteriosum (fig. 43G). 



Relations. — In front, the trunk of the pulmonary artery is covered by the remains of the 

 thyinu.s gl.'ind, and the f)ericanliuni. 'I'ho artery lies, at its commencement, behind the upper 

 margin of the third left chondro-sternal articulation. The right margin of the artery is behind 

 the second piece of sternum but the greater part of the vessel is behind the medial end of the 

 second intercostal space. 



Behind, it lies successively upon the ascending aorta and the left atrium. 



To tlie right are the ascending aorta, the right atrium, the right coronary artery, and the 

 qardiac nerves. 



