650 THE VEINS 



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 are formed by a separation of the 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. 



Veins have thinner walls than arteries, the difference in thickness being due to 

 the small amount of elastic and muscular tissues which the veins contain. The 

 superficial veins usually have thicker coats than the deep veins, and the veins of 

 the lower limb are thicker than those of the upper. 



The minute structure of these vessels has been described in the section on 

 General Anatomy. 



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 ramifications of the pulmonary artery, and, uniting 

 together, form a single trunk for each lobule. These branches, uniting succes- 

 sively, 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 unites with that from the 

 upper lobe, in most cases, 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 unfrequently 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 between 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 ; the left pass in front of the thoracic aorta with the left pulmo- 

 nary 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 limb, pelvis, and abdomen, which terminate in the inferior 

 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. 

 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. 



