96 



THE VASCULAR SYSTEM 



the vein would more likely possess the larger caliber ; the artery, 

 on the hand, would have the thicker wall. 



As compared with the arteries, the veins are notably deficient 

 in elastic and muscular tissue. In the wall of most veins the white 

 fibrous is in excess of all other tissues. For this reason the adven- 

 titia 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 recog- 

 nized 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 like- 

 wise 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 



FIG. 96. TRANSECTION OF AN ARTERIOLE AND VENULE. 

 x 250. (After Schafer.) 



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 consists of one, two, or more crescentic folds or redupli- 



