186 



HANDBOOK OF PHYSIOLOGY. 



The Veins. 



Distribution. -The venous system begins in small vessels which are 

 slightly larger than the capillaries from which they spring. These 

 vessels are gathered up into larger and larger trunks until they termi- 

 nate (as regards the systemic circulation) in the two venae cavae and the 

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

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

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



Fig. 160. 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 'etter is placed 

 in the lumen of the vessel, e. Endothelial cells with nuclei clearly visible ; these cells appear very 

 thick from the contracted state of the vessel. Outside it a double wavy line .narks the elastic 

 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 connective-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 (m) is seen to be much thinner than that of 

 the artery, x 350. (Klein and Noble Smith.) 



the heart; but, as a rule, their capacity exceeds by 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 pul- 

 monary arteries. The veins are found after death more or less collapsed, 

 and often contain blood. 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. 160). Thus, they possess outer, middle, and 

 internal coats. 



The outer coat is constructed of areolar tissue like that of the 



