594 THE BLOOD- VASCULAR SYSTEM. 



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, peroneal they exist generally in pairs, one lying on each side of the ves- 

 sel, and are called rence comites. The larger arteries as the axillary, subclavian, 

 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 spinal 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 mater, 

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

 continuous with the lining membrane of the veins. 



THE PULMONARY VEINS. 



The Pulmonary Veins return the arterial blood from the lungs to the left 

 auricle of the heart. They are four in number, two for each lung. The pulmo- 

 nary differ from other veins in several respects : 1. They carry arterial instead of 

 venous blood. 2. They are destitute of valves. 3. They are only slightly larger 

 than the arteries they accompany. 4. They accompany those vessels singly. 

 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 together, form one vessel for each lobule. These vessels, uniting suc- 

 cessively, 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, forming two trunks on each side, which open separately 

 into the left auricle. Occasionally they remain separate; there are then three 

 veins on the right side. Not urifrequently the two left pulmonary veins termi- 

 nate by a common opening. 



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

 behind, and the bronchi betiveen the two. 



At the root of the lung, the veins are in front, the artery in the middle, and 

 the bronchus behind. 



Within the pericardium, their anterior surface is 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 with the left pulmonary artery. 



THE SYSTEMIC VEINS. 



The systemic veins may be arranged into three groups : 1. Those of the head 

 and neck, upper extremity, and thorax, which terminate in the superior vena cava. 

 "2. Those of the lower extremity, abdomen, and pelvis, which terminate in the infe- 

 rior vena cava. 3. The cardiac veins, which open directly into the right auricle 

 of the heart. 



VEINS OF THE HEAD AND NECK. 



The veins of the head and neck may be subdivided into three groups: 1. The 

 veins of the exterior of the head and face. 2. The veins of the neck. 3. The 

 veins of the diploe and interior of the cranium. 



Veins of the Exterior of the Head and Face. 



The veins of the exterior of the head and face are the 



Frontal. Temporal. 



Supra-orbital. Internal Maxillary. 



Angular. Temporo-maxillary. 



Facial. Posterior Auricular. 

 Occipital. 



