MINUTE ANATOMY OF THE VEINS. 277 



longitudinal and an external transverse layer of muscular 

 fibres. 



MUSCLES. In accordance with the presence or absence of 

 muscles in the walls of the veins, these vessels are divided into 

 the muscular and the non-muscular. 



To the formor belong the veins of the pia and dura mater, 

 the veins of Breschet in the bones, the veins of the retina, the 

 lower portions of the veins of the trunk opening into the vena 

 cava superior, the external and internal jugular veins, the sub- 

 clavian veins, and the veins of the maternal portion of the 

 placenta. 



In accordance with the arrangement of the muscular tissue, 

 the veins may be divided into three groups ; namely, 



Veins with longitudinal muscles, as those of the pregnant 

 uterus. 



Veins with an internal layer of circularly, and an external 

 layer of longitudinally arranged muscular fibres of which 

 examples are found in the vena cava inferior, both in and 

 below the liver, the vena azygos, and the portal, hepatic, 

 internal spermatic, renal, and axillary veins. 



The third group includes veins possessing an internal and an 

 external longitudinal and a middle transverse layer of muscular 

 fibres. Amongst these are the iliac, crural, and popliteal veins, 

 the branches of the mesenteric veins, and the umbilical vein. 



A fourth group includes the veins with circular muscular 

 fibres, to which the veins of the upper and partly of the lower 

 extremities, the smaller veins of the neck, the internal mam- 

 mary vein, and the veins in the substance of the lungs belong. 



The arrangement of the muscles is thus seen to vary even 

 in the same vascular region. The middle-sized branches of the 

 mesenteric veins contain, for instance, two longitudinal muscular 

 layers with an intermediate circular layer, whilst, on the other 

 hand, the vena porta possesses a feebly developed internal layer 

 of circular fibres, and an external longitudinal layer of con- 

 siderable thickness. 



As regards the proportionate strength of the muscular coat, 

 the veins of the lower extremity and vena umbilicalis occupy 

 the first rank; and then follow in succession those of the abdo- 



