THE VEINS GENERALLY. 143 



capillaries ; by which minute tubes it is conveyed to the pul- 

 monary veins ; when the recipient branches increasing ulti- 

 mately unite into four, five, and occasionally six principal 

 trunks ; which all terminate in the left auricle of the heart. 



THE VEINS GENERALLY. 



Veins correspond wdth arteries in their common office of 

 caiTying the blood ; but with this difference ; that the arte- 

 ries distribute their blood from the heart to every part of 

 the body : whereas the veins bring back the blood from the 

 parts the arteries had carried it to, and restore it to the heart. 

 The veins may, therefore, be considered as canals arising 

 from minute tubes, called capillaries ; distributed over every 

 part of the body ; veins becoming larger, terminate in the 

 heart. Their coats are thin ; but tenacious and elastic. 

 Their number and calibre are greatly superior to that of the 

 arteries. We find them distributed in two orders ; a super- 

 ficial and a deep-seated, which communicate freely with each 

 other. The superficial order run immediately under the 

 skin ; the deep-seated accompany the arteries. As a guard 

 against obstruction, as in the jaws, their coats sometimes 

 enlarge into reservoirs {Fig 17 ./). The want of solidity 

 and resisting power in veins is compensated by internal 

 membranous projections, which form valves. A valve 

 appears to be a duplicature of the lining coat of the vein ; 

 raised into two and sometimes three folds : of these, one 

 edge adheres to the inner side of the vein ; the other is 

 free. By this formation the blood passing towards the 

 heart, keeps the valves continually open ; but when, by 

 pressure, the fluid is stopped in its course, the valves being 

 pressed backward, expand, and prevent its return. Valves 

 are not distributed equally throughout the venous trunks : 

 in some vessels, where the blood has to traverse upward, or 

 against gravity, they are numerous ; in others they are 

 entirely wanting. They are found in veins of the extremi- 

 ties, excepting the feet ; there are but few in the viscera ; 

 nor are there any in parts w^here the blood has to drop as it 

 w^ere into the heart. A good example of this is the jugu- 

 lars {Fig 17 . a) ; in which the valves are placed looking 

 towards the heart ; so that when the horse stands with 

 his head elevated, the blood only has to fall through the 



