Of the Veins. 



THE Veins are the vessels which serve to return 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 vessels of this kind, contain arterial blood, 

 which they return from the lungs to the left auricle of the heart. 



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

 right auricle of the heart. 



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

 abdominal cavity, returning the venous blood from the viscera of digestion, and 

 carrying it to the liver by a single trunk of large size, the vena portas. From 

 this organ, the same blood is conveyed to the inferior vena cava by means of the 

 hepatic veins. 



The veins, like the arteries, are found in nearly every tissue of the body ; they 

 commence by minute plexuses, which communicate with the capillaries, the 

 branches from which, uniting together, constitute trunks, which increase in size 

 as they pass towards the heart, from the termination of larger branches in them. 

 The veins are larger and altogether more numerous than the arteries ; hence, the 

 entire capacity of the venous system is much greater than the arterial, the pul- 

 monary veins excepted, which do not exceed in capacity the pulmonary arteries. 

 From the combined area of the smaller venous branches being greater than the 

 main trunks, it results, that the venous system represents a cone, the summit of 

 which corresponds to the heart ; its base, to the circumference of the body. In 

 form, the veins are not perfectly cylindrical, like the arteries, their walls being 

 collapsed when empty, and the uniformity of their surface being interrupted at 

 intervals by slight contractions, which indicate the existence of valves in their 

 interior. They usually retain, however, the same calibre as long as they receive 

 no neighboring branches. 



The veins communicate very freely with one another, especially in certain 

 regions of the body ; and this communication exists between the larger trunks 

 as well as between the smaller branches. Thus in the cavity of the cranium, and 

 between the veins of the neck, where obstruction of the cerebral venous system 

 would be attended with imminent danger, we find that the sinuses and larger 

 veins have large and very frequent anastomoses. The same free communication 

 exists between the veins throughout the whole extent of the spinal canal, and 

 between the veins composing the various venous plexuses in the abdomen and 

 pelvis, as the spermatic, uterine, vesical, prostatic, etc. 



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



The Superficial or Cutaneous Veins are found between the layers of superficial 

 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 inclosed in the same 

 sheath with those vessels. In 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 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 exist, for example, in the interior of the skull, 

 and are formed by a subdivision of the layers of the dura mater ; their outer coat 



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