190 THE BLOOD VASCULAR SYSTEM 



The cephalic, basilic, mesenteric, iliac, femoral, saphenous, uterine 

 and the dorsalis penis veins contain small longitudinal bundles in the 

 intima. Certain veins, e. g., saphenous, femoral, and popliteal, contain a 

 layer of longitudinal muscle in the intimal portion of the media. 



The adrenal veins contain, almost exclusively, longitudinal muscle 

 fibers, and in the renal, suprarenal, portal, splenic and phrenic veins and 

 the inferior vena cava these fibers form the greater portion of the tunica 

 adventitia. 



In the pulmonary veins the circular muscle fibers are highly developed, 

 the tunica media of these veins almost equaling in thickness that of the 

 corresponding pulmonary artery. As in other large veins, however, elastic 

 tissue is notably deficient in the tunica media of the pulmonary veins. 

 The muscle of the roots is partially of the cardiac type. 



The tunica media of the largest veins, e. g., the venae cavae, jugular, 

 innominate and subclavian, contain much fibrous and considerable elastic 

 tissue, the latter often forming incomplete membranous layers, which alter- 

 nate with the muscle, as in the arteries. Such structure is, however, limited 

 to the very largest of the veins. In the superior vena cava and the hepatic 

 vein the media is practically replaced by adventitia. 



The cranial veins (cerebral and meningeal) are conspicuous for the 

 almost entire absence of muscle from their walls, the large meningeal 

 sinuses being surrounded by a dense fibrous coat derived from the dura 

 mater, and lined by the usual endothelium. In the veins of the retina also, 

 and those of bones, a media is essentially lacking. 



The venous spaces of the erectile tissues have already been mentioned 

 as presenting to some extent the sinusoidal type of structure, these large 

 venous cavities possessing an extremely thin wall, in structure scarcely 

 more than endothelial lining. The afferent artery projects into the broad 

 vascular lumen, from which the efferent vein makes its exit. 



Comparison of the Larger and the Smaller Veins. Comparing 

 the larger with the smaller veins, the excess of elastic and muscular tis- 

 sue in the former is most noticeable. In the absence of specific stains, 

 elastic tissue can scarcely be recognized in the venules and smaller veins. 

 In the medium-sized vessels it is scanty, but is present in considerable 

 quantity in the largest vessels. 



The precapillary veins and venules contain scarcely any smooth 

 muscle. This tissue becomes more distinct in the small veins and 

 steadily increases proportionately to the size of the vessel ; in the largest 

 veins it is again relatively deficient. 



Comparison of the Vein with the Artery of Corresponding Size. 

 The lumen of any given artery is always much smaller than the total 



