90 



THE VASCULAR SYSTEM 



elastic membrane, the homologue of the complete elastic coat or 

 fenestrated coat of Henle, which is found only in larger vessels. 



The smooth mus- 

 cle, on the other 

 hand, increases in 

 relative amount 

 from the larger to 

 the smaller arteries. 

 While in the largest 

 vessels it forms not 

 more than one- 

 third, in the arte- 

 rioles it represents 

 about three-fourths 

 of the arterial wall. 

 In the largest 

 arteries the adven- 

 titia is relatively 

 very thin. That of 

 the medium sized 

 vessels is somewhat 

 thicker, and the ra- 

 tio of connective tis- 

 sue as found in the 

 wall of these vessels 

 remains fairly con- 

 stant down to the 

 arterioles. In the 

 wall of the precapil- 

 lary arteries con- 

 nective tissue is 

 very scanty. 



CAPILLARIES. 

 The capillaries 

 are minute tubes, 5 

 to 13/x in diameter, 

 which, in nearly ail 

 the tissues of the 

 body, connect the 

 arteries with the 

 veins. Their wall 



FIG. 91. THK CAPILLARY NETWORK CONNECTING AN AR- 

 TERIOLE AND VENULE OF THE OMENTUM OF A YOUNG 



RABBIT. 



The blood vessels have been injected. The discolora- 

 tions at I and I are due to the presence of lacteals beneath 

 the endothelium ; at V and V these are surrounded by the 

 capillary network, a, arteriole; v, venule. Considerably 

 magnified. (After Eanvier.) 



