678 



HUMAN ANATOMY. 



Intitna 



Longitudinal 

 bundles of 

 striated 

 muscle 



Media 

 containing 

 striated 

 muscle 



Lack of muscle within the media is often compensated by an unusual develop- 

 ment of such tissue in the adventitia; in some large veins, as in the hepatic portion of 



the inferior cava, su- 

 FiG. 639. perior mesenteric, or 



external iliac, the in- 

 ner half or two-thirds 

 of the outer coat is 

 occupied by robust 

 bundles of longitudi- 

 nally arranged muscle. 

 In some cases, how- 

 ever, as in the renal 

 and portal veins, the 

 longitudinal muscle in- 

 vades the entire thick- 

 ness of the adventitia, 

 or, as in the supra- 

 renal vein, the muscle 

 of the outer tunic may 

 include both circular 

 and longitudinal layers. 

 The walls of the 

 small veins (less than 

 .4 mm. in diameter) 

 consist of only endo- 

 thelium and connective 

 tissue. The latter rep- 

 resents a relatively ro- 

 bust adventitia and a 

 feebly developed me- 

 dia, muscle-fibres being 

 wanting. Traced tow- 

 ards the capillaries, the connective tissue gradually diminishes until the endothelial 

 coat alone remains. In passing into veins of medium size, at first the muscle-cells are 

 short and scattered and only 



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Adventitia 



"^^ 



Transverse section of pulmonary artery near its root, 

 showing striated muscle. X 150- 



Fig. 640. 



Intmia 



Media 



partly encircle the tube. Far- 

 ther along the elastica appears 

 in the form of delicate fibres 

 and net-works that increase in 

 size and density as the muscu- 

 lar tissue becomes more pro- 

 nounced. It is worthy of 

 mention that certain veins, no- 

 tably those of the brain and pia 

 mater, the dural sinuses, and 

 the blood-spaces of cavernous 

 tissue, are usually entirely 

 devoid of muscle, although 

 in the walls of some of the 

 larger cerebral veins, small 

 strands of such tissue occur. 



The Capillaries.— The 

 most favorable arrangement 

 for efificient nutrition is mani- 

 festly one insuring the passage 

 of the blood-stream at a re- 

 duced rate of speed in inti- 

 mate relations with the tissue-elements.^ These requirements are met in the capil- 

 laries whose collectively increased calibre and thin walls favor slowing of the blood- 



Adventitia 



Transverse section of vein of medium size. X 250. 



