THE VEINS. 



THE Veins convey the blood from the capillaries of the different parts of the 

 body to the heart. They consist of two distinct sets of vessels, the pulmonary 

 and systemic. 



The Pulmonary Veins, unlike other veins, contain arterial blood, which they return 

 from the lungs to the left atrium of the heart. 



The Systemic Veins return the venous blood from the body generally to the 

 right atrium of the heart. 



The Portal Vein, an appendage to the systemic venous system, is confined to 

 the abdominal cavity, and returns the venous blood from the spleen and the viscera 

 of digestion to the liver. This vessel ramifies in the substance of the liver and there 

 breaks up into a minute network of capillary-like vessels, from which the blood 

 is conveyed by the hepatic veins to the inferior vena cava. 



The veins commence by minute plexuses which receive the blood from the capil- 

 laries. The branches arising from these plexuses unite together into trunks, and 

 these, in their passage toward the heart, constantly increase in size as they receive 

 tributaries, or join other veins. The veins are larger and altogether more numerous 

 than the arteries; hence, the entire capacity of the venous system is much greater 

 than that of the arterial; the capacity of the pulmonary veins, however, only 

 slightly exceeds that of the pulmonary arteries. The veins are cylindrical like the 

 arteries; their walls, however, are thin and they collapse when the vessels are 

 empty, and the uniformity of their surfaces is interrupted at intervals by slight 

 constrictions, which indicate the existence of valves in their interior. They com- 

 municate very freely with one another, especially in certain regions of the body; 

 and these communications exist between the larger trunks as well as between the 

 smaller branches. Thus, between the venous sinuses of the cranium, and between 

 the veins of the neck, where obstruction would be attended with imminent danger 

 to the cerebral venous system, large and frequent anastomoses are found. The 

 same free communication exists between the veins throughout the whole extent 

 of the vertebral canal, and between the veins composing the various venous plexuses 

 in the abdomen and pelvis, e. g., the spermatic, uterine, vesical, and pudendal. 



The systemic venous channels are subdivided into three sets, viz., superficial 

 and deep veins, and venous sinuses. 



The Superficial Veins (cutaneous veins) are found between the layers of the 

 superficial fascia immediately beneath the skin; 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 

 sheaths 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 vessel, 

 and are called venae comitantes. The larger arteries such 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 bones. 



Venous Sinuses are found only in the interior of the skull, and consist of canals 

 formed by a separation of the two layers of the dura mater; their outer coat con- 

 sists of fibrous tissue, their inner of an endothelial layer continuous with the lining 

 membrane of the veins. 



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