CH. XIX.] 



THE VEINS 



219 



Rl 



coronary veins, which enter the right auricle, and (as regards the 



pulmonary circulation) in four pulmonary veins, which enter the left 



auricle. The total capacity of the veins diminishes as they approach 



the heart ; but, as a rule, their capacity is two or three times that 



of the corresponding arteries. The 



pulmonary veins, however, are an 



exception to this rule, as they do not 



exceed in capacity the pulmonary 



arteries. The veins are found after 



death more or less collapsed, owing 



to their want of elasticity. They are 



usually distributed in a superficial 



and a deep set which communicate 



frequently in their course. 



Structure. In structure the coats 

 of veins bear a general resemblance 

 to those of arteries (fig. 200). Thus, 

 they possess outer, middle, and in- 

 ternal coats. 



(a) The outer coat is constructed 

 of areolar tissue like that of the 

 arteries, but it is thicker. In some 

 veins it contains muscular fibres, 

 which are arranged longitudinally. 



(2>) The middle coat is consider- 

 ably thinner than that of the arteries ; 

 it contains circular unstriped mus- 

 cular fibres, mingled with a few 

 elastic fibres and a large proportion 

 of white fibrous tissue. In the large 

 veins, near the heart, namely, the 

 venae cavce and pulmonary veins, the 

 middle coat is replaced, for some 

 distance from the heart, by circularly 

 arranged striped muscular fibres, con- 

 tinuous with those of the auricles. 

 The veins of bones, and of the central 

 nervous system and its membranes 

 have no muscular tissue. 



(c) The internal coat of veins 

 membrane, which may be absent 

 endothelium is made up of cells elongated in the direction of the 

 vessel, but wider than in the arteries. 



Valves. One main distinction between arteries and veins is 

 the presence of valves in the latter vessels. The general construc- 



Fio. 200. Transverse section through a 

 small artery and vein of the mucous 

 membrane 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, Endo- 

 thelial 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 

 layer of the tunica intima. m, Tunica 

 media, consisting of unstriped muscular 

 fibres circularly arranged ; their nuclei 

 are well seen, a, Part of the tunica 

 adventitia showing bundles of connec- 

 tive-tissue fibre in section, with the 

 circular nuclei of the connective-tissue 

 corpuscles. This coat gradually merges 

 into the surrounding connective tissue, 

 v. In the lumen of the vein. The other 

 letters indicate the same as in the 

 artery. The muscular coat of the vein 

 (TO) is seen to be much thinner than 

 that of the artery, x 350. (Klein 

 and Noble Smith.) 



has 

 in 



a very thin 

 the smaller 



fenestrated 

 The 



veins. 



