THE PULMONARY VEINS 709 



and a very strongly developed adventitia. The intima is a connective-tissue layer containing 

 a small number of elastic fibres and lined with endothelium. The internal elastic lamina is 

 usually poorlv developed. The valves are duplifations of the intima. The media consists 

 chiefly of white fibrous tissue containing some circular muscle fibres and some fine elastic- 

 fibres. In some veins the media is thoroughly well developed (veins of the lower extremities), 

 in others it is practically absent (veins of the retina, of the pia, of bone, the superior vena 

 cava). The adventitia is dense and strong, and is composed of fibroelastic tissue and non- 

 striated muscle fibres longitudinally placed. Fig. 492 shows a transverse section of part of the 

 wall of a vein. The large veins and the veins of medium size possess vasa vasorum, in the 

 adventitia and to some extent in the media. The walls of veins contain vasomotor nerves. 

 Lymph capillaries often surround the smaller bloodvessels and sometimes by spaces lined 

 with endothelium and which are in communication with the lymphatic system; these spaces 

 are called perivascular lymph spaces. 



The systemic veins are subdivided into three sets superficial, deep, and sinuses. 



The Superficial or Cutaneous Veins are found between the layers of the super- 

 ficial fascia, immediately beneath the integument; they return the blood from 

 these structures, and communicate with the deep veins by perforating the deep 

 fascia. 



The Deep Veins accompany the arteries, and are usually enclosed in the same 

 sheath with those vessels. With the smaller arteries as the radial, ulnar, brachial, 

 tibial, and peroneal they exist generally in pairs, one lying on each side of the 

 vessel, and are called venae comites. The larger arteries as the axillary, sub- 

 clavian, popliteal, and femoral have usually only one accompanying vein. In 

 certain organs of the body, however, the deep veins do not accompany the arteries; 

 for instance, the veins in the skull and vertebral canal, the hepatic veins in the 

 liver, and the larger veins returning blood from the osseous tissue. 



Sinuses are venous channels which, in their structure and mode of distribution, 

 differ altogether from the veins. They are found only in the interior of the skull, 

 and consist of channels formed by a separation of the two layers of the dura, their 

 outer coat consisting of fibrous tissue, their inner of an endothelial layer con- 

 tinuous with the lining membrane of the veins. 



THE PULMONARY VEINS (V. PULMONALES) (Fig. 493). 



The pulmonary veins return the arterialized blood from the lungs to the left 

 auricle of the heart. They are four in number, two for each lung, and are destitute 

 of valves. They commence in a capillary network upon the walls of the air-cells, 

 where they are continuous with the capillary ramifications of the pulmonary artery. 

 and uniting, form one vessel for each lobule. These vessels, uniting successively 

 form a single trunk for each lobe, three for the right and two for the left lung. 

 The vein from the middle lobe of the right lung generally unites with that from 

 the upper lobe. Thus, two trunks are formed on each side, which open separately 

 into the left auricle. Occasionally the three veins on the right side remain sepa- 

 rate. Not infrequently the two left pulmonary veins terminate by a common 

 opening. 



Within the lung, the branches of the pulmonary artery are in front, the veins 

 behind, and the bronchi between the two. 



At the root of the lung, the upper pulmonary vein lies in front of and a little below 

 the pulmonary artery; the lower is situated below the other structures in the lung 

 root, and on a plane posterior to the upper vtein; behind the pulmonary artery is 

 the bronchus. 



Within the pericardium, their anterior surfaces are invested by the serous layer 

 of this membrane. The right pulmonary veins pass behind the right auricle and 

 ascending aorta and superior vena cava; the left pass in front of the thoracic aorta. 



