172 THE CIRCULATION OF THE BLOOD 



branes, for example in the conjunctiva and in the cutis vera, the capillary 

 vessels are much larger than in the brain and the interspaces narrower, 

 namely, not more than three or four times wider than the vessels. In the 

 periosteum and in the external coat of arteries the meshes are much larger, 

 their width being about ten times that of the vessels. It may be held as a 

 general rule that the more active the functions of an organ are, the more 

 vascular it is. 



The Veins. The venous system begins in small vessels which are 

 slightly larger than the capillaries from which they spring. These vessels 



FIG. 149. Transverse Section through a Small Artery and Vein of the Mucous Mem- 

 brane of a Child's Epiglottis; the artery is thick- walled and the vein thin-walled. A, 

 Artery; the letter is placed in the lumen of the vessel, e, Endothelial cells with nuclei 

 clearly visible; these cells appear very thick from the contracted state of the vessel. Outside 

 it a double wavy line marks the elastic tunica intima. m, Tunica media consisting of 

 unstriped muscular fibers circularly arranged; their nuclei are well seen, a, Part of the 

 tunica adventitia, showing bundles of connective-tissue fiber in section, with the circular 

 nuclei of the connective-tissue corpuscles. This coat gradually merges into the surrounding 

 connective tissue. V, The lumen of the vein. The other letters indicate the same as in 

 the artery. The muscular coat of the vein, m, is seen to be much thinner than that of the 

 artery. X 350. (Klein and Noble Smith.) 



are gathered up into larger and larger trunks until they terminate in the two 

 venae cavae and the coronary vein which enter the right auricle, and in four 

 pulmonary veins which enter the left auricle. The total capacity of the 

 veins diminishes as they approach the heart; but their capacity exceeds by 

 two or three times that of their corresponding arteries. The pulmonary 

 veins, however, are an exception to this rule. The veins are found after 



