136 HAND-BOOK OF PHYSIOLOGY. 



the capacity of the veins exceeds by several times (twice or three times) 

 that of their 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 as a rule to be more 

 or less collapsed, and often to contain blood. The veins are usually dis- 

 tributed in a superficial and a deep set which communicate frequently in 

 their course. 



Structure. In structure the coats of veins bear a general resem- 

 blance to those of arteries (Fig. 118). Thus, they possess an outer, 



TIG. 118. Transverse section through a small artery and vein of the mucous membrane of a 

 Child's epiglottis: the contrast between the thick- walled artery and the thin- walled vein is well shown. 

 A. Artery, the letter is placed in the lumen of the vessel, e. Endothelial cells with nuclei clearly vis- 

 ible: these cells appear very thick from the contracted state of the vessel. Outside it a double wavy 

 line marks the elastic tunica intima. ra. Tunica media forming the chief part of arterial wall and 

 consisting of unstriped muscular fibres circularly arranged: their nuclei are well seen, a. Part of 

 the tunica adventitia showing bundles of connective-tissue fibres 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 mus- 

 cular coat of the vein (m) is seen to be much thinner than that of the artery. X 350. (Klein and 

 Noble Smith.) 



middle, and internal coat. The outer coat is constructed of areolar tissue 

 like that of the arteries, but is thicker. In some veins it contains mus- 

 cular fibre-cells, which are arranged longitudinally. 



The middle coat is considerably thinner than that of the arteries; and, 

 although it contains circular unstriped muscular fibres or fibre-cells, these 

 are mingled with a larger proportion of yellow elastic and white fibrous 

 tissue. In the large veins, near the heart, namely the vencs caves and 

 pulmonary veins, the middle coal; is replaced, for some distance from the 

 heart, by circularly arranged striped muscular fibres, continuous with 

 those of the auricles. 



