NORMAL HISTOLOGY. 



FIG. 122. 



of the media is very scant or altogether wanting in a number of 

 veins, including the thoracic part of the vena cava, the internal and 

 external jugular veins, the veins of the pia and dura, of the retina, 

 of bone, and of the corpora cavernosa. Certain veins possess longi- 

 tudinal muscular bundles in the inner part of the media ; such are 

 the mesenteric, umbilical, iliac, and femoral. 



The adventitia, often the thickest coat of the vein, consists of 

 stout net-works composed of bands of fibro-elastic tissue ; in some 

 veins additional bundles of plain muscle occur within this tunic. 



Among the venous trunks possessing well- 

 marked, longitudinally arranged muscu- 

 lar tissue in the external coat are the 

 abdominal cava, azygos, hepatic, portal, 

 splenic, axillary, superior mesenteric, 

 renal, spermatic, and external iliac veins. 

 The veins of the gravid uterus contain 

 muscular tissue in all the coats, the prin- 

 cipal bundles running longitudinally. 



The valves with which many veins are 

 provided consist of crescentic folds of the 

 inner tunic of the vessel, strengthened by 

 additional fibro-elastic tissue ; in some 

 instances the muscular bundles extend for 

 a short distance into the valve. The base 

 or the attached margin of the valve is often 

 its thinnest part, the free edges being 

 somewhat thickened. The striated car- 

 diac muscular tissue is continued for a 

 short distance in the walls of those parts 



of the venae cavae and of the pulmonary veins immediately adjoining 

 the heart ; the explanation of this fact is found in the derivation of 

 these portions of the vessels from the tissues of the primitive heart- 

 tube. 



THE CAPILLARIES. 



The capillaries establish the only communication, with few excep- 

 tions, between the arteries and the veins, and, further, provide the 

 intimate anatomical relation between the nutritive current and the 

 tissues of the body necessary for the maintenance of the integrity 

 and functional activity of the various organs. As exceptions to the 

 usual intervention of the capillaries between the arterial and venous 

 radicles, the direct communication between these vessels existing in 

 the erectile tissue of the genital organs, in the spleen, and in some 

 parts of the peripheral circulation, as in the tips of the fingers and 

 toes and of the nose, may be mentioned. 



Section of human vein of medium 

 size: /, M, and A, respectively 

 intiina, media, and adventitia. 



