THE PULMONARY ARTERY 



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The latter is nourished by the blood that flows over it. They may arise from the vessels to 

 which they are distributed or take origin from an adjacent vessel. The blood is returned from 

 the walls of the vessels by small veins. 



Lymphatics. Distinct lymphatic vessels may exist in the adventitia, but are represented 

 by lymph spaces in the other coats. Lymph capillaries often surround small bloodvessels, or a 

 small bloodvessel may lie in a perivascular lymph space. 



Nerves. Arteries are supplied with nerves, myelinie and amyelinic. A network of nerve 

 fibres may surround a vessel, and usually capillaries are so surrounded. In the arteries a 

 network of nerves exists in the media. These nerves supply the muscle fibres, and are called 

 vasomotor nerves. According to Stohr, nerve endings are found in the endothelium of the 

 capillaries, giving them the power of contractility. 



The Arterial Sheath (vaaina JY/W-) surrounds the artery. It is composed of connective 

 tissue, and is attached to the vessel at numerous points by fibrous tissue. 



FIG. 429. Capillaries from the 

 mesentery of a guinea-pig after 

 treatment with a solution of nitrate 

 of silver, a. Cells, b. Their nuclei. 



FIG. 430. Finest vessels on the arterial side. From the human 

 brain. Magnified 300 times. 1. Small artery. 2. Transition vessel. 

 3. Coarser capillaries. 4. Finer capillaries, a. Structureless mem- 

 brane still with some nuclei, representative of the tunica adven- 

 titia. 6. Nuclei of the muscle fibre cells. c. Nuclei within the 

 small artery; perhaps appertaining to an endothelium. d. Nuclei 

 in the transition vessels. 



THE PULMONARY ARTERY (A. PULMONALIS) (Figs. 432, 436). 



The pulmonary artery conveys the venous blood from the heart to the lungs. 

 It is a short, wide vessel, about two inches (5 cm.) in length and one and one- 

 fifth inches (30 mm.) in diameter, arising from the left side of the base (conus 

 arferiosus) of the right ventricle, in front of the aorta. It extends obliquely 

 upward and backward, passing at first in front of and then to the left of the 

 ascending aorta, as far as the under surface of the arch, where it divides, about 

 on a level with the intervertebral substance between the fifth and sixth thoracic 

 vertebrae, into right and left branches of nearly equal size. 



Relations. The whole of the vessel is contained, together with the ascending aorta, in the 

 pericardium. It is enclosed with the aorta in a single tube of the serous pericardium, which is 

 continued upward upon them from the base of the heart and connects them together. The 

 fibrous layer of the pericardium becomes gradually lost upon the external coats of its two branches. 

 In front, the pulmonary artery is separated from the anterior extremity of the second left inter- 

 costal .space by the pleura and left lung, in addition to the pericardium; it rests at first upon 



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