Of the Veins. 



THE Veins are the vessels which serve to return the blood from the capilla- 

 ries of the different parts of the body to the heart. They consist of two distinct 

 sets of vessels, the pulmonary arid 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 portal. 

 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 which have their commencement in these plexuses unite together 

 into trunks, and these, in their passage towards the heart, constantly increase 

 in size as they receive branches, and join other veins similar in size to them- 

 selves. 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 pulmonary 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 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 would be attended with 

 imminent danger to the cerebral venous system, we find that the sinuses and 

 larger veins have large and very frequent anastomoses. The same free com- 

 munication 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 superfi- 

 cial 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 vense comites. The larger arteries, as the axillary, sub- 

 clavian, popliteal, and femoral, and 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. 



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