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 systonic. 



The Pulmonary Veins are concerned in the circulation in the lungs. Unlike 

 other vessels of this kind, they contain arterial blood, which they return from the 

 lungs to the left auricle of the heart. 



The Systemic Veins are concerned in the general circulation ; they 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 portce. This 

 vessel ramifies in the substance of the liver and breaks up into a minute network 

 of capillaries. These capillaries then re-collect to form the hepatic veins, by which 

 the blood is conveyed to the inferior vena cava. 



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

 They commence by minute plexuses which receive the blood from the capillaries. 

 The branches which have their commencement in 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 pulmonary veins 

 excepted, which only slightly 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 perfectly cylindrical, like the arteries, their walls being collapsed when 

 empty, and the uniformity of their surface being interrupted at intervals by slight 

 constrictions, 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 immi- 

 nent danger to the cerebral venous system, 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 pel- 

 vis, as the spermatic, uterine, vesical, and prostatic. 



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 will be described in the section on 

 General Anatomy. 



The systemic veins arc subdivided into three sets : superficial, deep, and sinuses. 



The Superficial or Cutaneous Veins are found between the layers of the super- 

 ficial fascia, immediately beneath the integument; they return the blood from 

 these structures, and communicate with the deep veins by perforating the deep 

 fascia. 



