VEINS 95 



In certain tissues the veins present noticeable departures from 

 the typical structure. Longitudinal muscle fibres are found in 

 many of the larger veins of the abdominal and thoracic cavities. 



The adrenal veins contain, almost exclusively, longitudinal 

 muscle fibres, and in the renal and phrenic veins and the vena 

 cava these fibres form the greater portion of the tunica adventitia. 



In the pulmonary veins the circular muscle fibres 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 vessels. 



The tunica media of the largest veins, e. g., the vena cava, con- 

 tains much fibrous and considerable elastic tissue, the latter often 

 forming incomplete membranous layers, which alternate with the 

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

 the very largest of the veins. 



The cranial veins are conspicuous for the almost entire absence 

 of muscle from their walls, the large meningeal sinuses being sur- 

 rounded by a dense fibrous coat derived from the dura mater, 

 and lined by the usual endothelium. 



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 an 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 mus- 

 cular tissue 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 lumen of its venas comites, the ratio being about one to 

 three. Hence, of any two vessels in close proximity to each other, 



