122 NORMAL HISTOLOGY. 



the character of endothelium. The capillaries are 

 endothelial tubes. This layer of endothelial cells, 

 which alone forms the walls of the capillaries, is 

 found lining all the other blood-channels, arteries, 

 and veins, as well as the heart. In the blood-vessels 

 it is called the intima. 



If, now, we follow the capillaries in a direction 

 toward the arteries, we find that the connective 

 tissue in which they lie is arranged in the form of a 

 thin layer along their walls. This layer, which is 

 also present in all arteries and veins, is called the 

 adventitia. Almost as soon as we find the adven- 

 titia, we notice another layer between it and the 

 intima, formed of a single row of smooth-muscle 

 cells, or of scattered cells, wound transversely or 

 obliquely around the vessel. This layer is called 

 the media or musculosa } and a vessel having these 

 three simple layers in its walls is called an artcriole. 

 In these three layers, intima, media, and adventitia, 

 we have the types of all the layers which occur in 

 the walls of the largest and most complicated blood- 

 vessels. The individual layers become, indeed, more 

 complex in structure ; but, with the exception of 

 elastic elements, no new tissues appear. 



Turning now to a larger artery the radial, for 

 example and examining the various layers in its 

 walls, we find that the intima is no longer formed 

 of a simple endothelial tube, but that outside of 

 this a new layer has appeared, composed of ill 



