VEINS. 



129 



All the medium-sized and large blood -vessels are supplied 

 with small vessels (vasa vasorurn) which supply their walls 

 with blood. They run in the adventitia, and only to a small 



FIG. <) 



Endothelial layer 

 Medi 



Adventitia < 



Vas ra.s/.s _ . 





Part of a cross-section of the vena eava inferior of a clog. X 150. 



extent in the media. They never reach to the intima. Small 

 blood-vessels are often surrounded by lymph capillaries, and 

 sometimes by endothelium-lined spaces which are in communi- 

 cation with the lymphatic system. These are called perivas- 

 cular lymph spaces, and are found in the central nervous 

 system, bones, etc. 



The vessel walls are also supplied with nerves. Medullated 

 and non-medu Hated fibres form a network in the media, and 

 may end in any of the accessory coats. Capillaries are sur- 

 rounded usually by a fine network of nerve filaments. 



(d) The Heart. 



The heart is a much complicated part of the circulatory 

 system, with walls that are made up of three main layers : 



