VEINS 



93 



VEINS. The blood having passed the capillaries, enters the 

 smallest radicals of the venous system, the precapillary venules, and 

 passes thence through the venules to the larger veins. The pro- 

 gressive increase in the caliber of these successive vessels is ac- 

 companied by a corresponding increase 

 in the thickness of their wall. Thus, 

 while the endothelial tube alone com- 

 poses the capillary wall, the endothe- 

 lium of the precapillary venule is en- 

 circled by a delicate connective tissue 

 membrane. In the venule occasional 

 smooth muscle fibres are added to the 

 wall of the smaller vessel, and in the 

 vessels of this caliber the fibrous tis- 

 sues have been so increased that the 

 vascular wall, as in the artery, can be 

 said to possess three coats. 



The wall of the precapillary venule 

 consists of the endothelial lining, 

 which is surrounded by a very delicate 

 connective tissue membrane in which 

 are very few elastic and white fibres. 



In the venule the tunica intima 

 consists of little more than the endo- 

 thelial lining. Its media and adventitia are not as yet distinctly 

 diiferentiated, the former being distinguished only by the incom- 

 plete layer of circularly disposed smooth muscular fibres. The 

 extremely thin adventitia is composed almost wholly of white 

 fibres, the greater part of whicli are circularly disposed. Very few 

 elastic fibres occur even in vessels of this size. 



In the small veins the three coats are fairly distinct, the vas- 

 cular wall being, however, much thinner than in the artery of 

 corresponding size. 



The endothelium of the tunica intima is supported by a very 

 delicate connective-tissue membrane which as yet contains but 

 few elastic fibres. 



The tunica media consists of a thin layer of circularly arranged 

 smooth muscle fibres intermingled with a delicate fibrous tissue ; 

 elastic fibres are relatively scarce. 



The adventitia, though considerably the thickest of three 

 coats, is as yet a thin membrane. It consists of fibrous connective 



PRECAPILLARY ARTERI- 

 OLE AND VENULE. 



The lighter nuclei are those 

 of the endothelium. The darker 

 nuclei in the venule are in con- 

 nective tissue cells ; in the arte- 

 riole they are in the muscle cells. 

 A, venule ; B, arteriole. Partly 

 diagrammatic. Highly magnified. 



