708 



THE VASCULAR 8YS7EMS 



branches which have their commencement in these plexuses unite 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 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 tributaries 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 periphery of the body. In form the veins are cylindrical, 

 like the arteries, their walls being collapsed when empty, and the uniformity of 

 their surface being interrupted at intervals by slight dilatations, which indicate 

 the existence of valves in their interior (Fig. 489). They usually retain, however, 

 about the same calibre as long as they receive no tributaries, but not so uniformly 

 as do the arteries. 



The veins communicate very freely with one another (Fig. 491), especially in 

 certain regions of the body, and this communication exists between the larger 

 trunks as well as between the smaller 

 tributaries. Thus, in the cavity of the 

 cranium and between the veins of the 

 neck, where obstruction would be at- 

 tended with imminent danger to the 

 cerebral venous system, we find that 

 the sinuses and larger veins have large 



Endothelial 

 layer 

 Media 1 



Adventitia 



FIG. 491. The venous circle of Braune (schematic). 

 The arrows indicate the direction of the blood current. 

 (Poirier and Charpy.) 



I'as vasis 



FIG. 492. Part of a cross-section of the vent 

 cava inferior of a dog. (Szymonowicz.) 



and very frequent anastomoses (Fig. 490). The same free communication exists 

 throughout the whole extent of the vertebral canal, and between the veins 

 composing the various venous plexuses in the abdomen and pelvis, as the sper- 

 matic, 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. 



Histology of the Veins. As previously stated, capillaries enter into venules or post- 

 capillary veins. The venules empty into larger veins. Vein walls are much thinner than 

 arterial walls. A vein has a much thinner media and much less elastic tissue than an artery, 



