VEINS 191 



lumen of its venae comites (usually two in the case of the smaller arter- 

 ies, one vena comes in the case of the medium-sized), the ratio being 

 about one to three. Hence, of any two vessels in close proximity to 

 each other, the vein would more likely possess the larger caliber; the 

 artery, on the other hand, would have the thicker wall. 



As compared with the arteries, the veins are notably deficient in elas- 

 tic and muscular tissue. In the wall of most veins the white fibrous is 

 in excess of all other tissues. For this reason the adventitia is almost 

 invariably the thickest of the three coats of the vein, whereas in the 

 artery the media is always the thickest coat. 



The internal elastic membrane, which can be readily recognized even 

 in the smaller arteries, is limited to the large veins. Alternating layers 

 of elastic and muscular tissues are to be seen even in the medium-sized 

 arteries, but this arrangement is likewise confined to the largest of the 

 veins. 



The wall of the vein as a whole is much thinner in proportion to its 

 lumen than that of the corresponding artery ; it is also less rigid. For 

 this reason the wall of the vein is much more likely to collapse after 

 death than is the thicker and more rigid arterial wall. Because of the 

 preponderance of muscle in the wall of the artery its contraction in 

 rigor mortis is more powerful than that of the vein ; the vein therefore is 

 apt to be distended with blood while the artery contains but little. A 

 certain number of blood-cells can usually be found in almost any type of 

 blood-vessel. 



Valves occur at intervals of considerable length along the course of 

 the larger veins. These are not found in the arteries. Each valve con- 

 gists of one, usually two, and occasionally more crescentic folds or redupli- 

 cations of the tunica intima between which is a slightly increased amount 

 of connective tissue, the elastic fibers of which are more abundant on the 

 side next the lumen. The valves therefore are suspended free in the 

 lumen of the vessel and are covered on either side with a layer of endo- 

 thelium which is continuous with that lining the vein. 



The valves open with and close against the blood current. They 

 occur generally distal to the point of entrance of venous tributaries. They 

 are more abundant in the veins of the extremities and are lacking in the 

 superior and inferior venae cava3, in the hepatic, portal, renal, uterine, 

 pulmonary, umbilical, cerebral and meningeal veins, in the veins of bones, 

 and in veins of less than 2 millimeters diameter. They obviously assist 

 the flow of blood to the heart against the influence of gravity. The gen- 

 eral absence of valves in the veins of the abdomen and thorax, and their 



