THE CIRCUT.ATORV SYSTEM 163 



disposed smooth mustlc cells. In most veins the amount of muscle 

 is comparative!}- small and the cells are distributed in ^n-oui).-- through 

 the connective tissue. The media is thickest in the veins of the lower 

 extremities, especially the ])opliteal. and in the veins of the skin. In 

 the veins of the head and abdomen the media is very thin, while in 

 the subclavian and superior vena ca\a and in the veins of bones, of 

 the pia mater, dura mater, and retina, there is an almost entire 

 absence of mecha. 



The ADVENTITIA is well developed in i)roportion to the media and 

 forms the bulk of the vessel wall. It consists of mixed fibrous and 

 elastic tissue and usually contains along its inner margin small 

 bundles of longitudinally disposed smooth muscle cells. 



Arteries are as a rule empty after death, while veins contain blood. 

 The absence of much elastic tissue in the walls of the veins prevents 

 any such extensive post-mortem contraction as occurs in the arteries. 

 Veins tend to collapse after death, but are usually prevented from 

 doing so by the presence of blood in them. 



In the iliac and femoral veins, longitudinally disposed muscle occurs in the 

 inner part of the media. The umbilical vein, like the corresponding artery, has 

 three distinct muscular coats. Longitudinal muscle fibres are present in the 

 adventitia of the superior vena cava (hepatic and abdominal portion) and of 

 the portal and hepadc veins. In the upper portion of the inferior vena cava, 

 in the superior vena cava, the jugular, innominate, and subclavian, there is little 

 muscle tissue in any of the coats, while in the veins of the brain and its mem- 

 branes, the retina, the placenta and the bones, no muscle is present. 



Vasa Vasorum. — Medium and large arteries and veins are sup- 

 pUed with small nutrient vessels — vasa vasorum. These vessels run 

 in the adventitia, small branches penetrating the media (Figs. 95 

 and 96). 



Lymph channels are found on the outer surface of many blood- 

 vessels. Some of the smaller vessels are surrounded by spaces Uned 

 by endothelium — perivascular lymph spaces. These communicate 

 with the general lymphatic system. 



Nerves. — The walls of the blood-vessels are suppHed with both 

 medulla ted and non-medullated fibres. The latter are axones of 

 sympathetic neurones. As these nerves control the caHbre of the 

 vessels they are known as vasomotor nerves. They form plexuses 

 in the adventitia, from which are given off branches which pene- 

 trate the media and terminate on the muscle cells. The medul- 

 lated fibres are the peripheral arms of spinal or cranial gangHon cells. 



