THE ARTERIES 



117 



muscle. Ill the larfjer branches the proportion of elastic fibers in 

 the media increases. The intima is often ])o()rly preserved, the 

 endothelial cells bein^ indicated by their nuclei which protrude into 

 the lumen. The subendothelial connective tissue is generally 

 inconspicuous but is better seen in the larger \essels of this class. 

 The internal elastic membrane appears as a clear, wavy line 

 and the thick media shows numerous layers of smooth muscle 

 separated by connective tissue. In larger vessels, elastic fibers 

 appear more prominently in the media. The adventitia is as 

 thick as the media or may be thicker, with elastic fibers becoming 

 more numerous toward the media. 



Fig. 67. — A photograph of a small 

 artery of the cat, showing internal 

 and external elastic membranes out- 

 lining the muscular and elastic media. 



Fig. 68. — Aorta of the dog, 

 fixed in Bouin's fluid and 

 stained with hematoxylin 

 and eosin, showing the heavy 

 elastic media and fibrous 

 adventitia. , 



Large or Elastic Arteries.— These ^TSsels, such as the aorta and 

 the pulmonary arteries, have a relati^•ely thick wall which appears 

 thin in view of the large size of the lumen. When compared with 

 a medium-sized artery the muscle has decreased in amount and there 

 is a greater supply of elastic fibers. The intima is thicker due 

 to the presence of more subendothelial connective tissue. The 

 internal elastic lamina is not evident and in the media are many 

 layers of fenestrated elastic membranes which are seen best when 

 orcein or resorcin fuchsin is used as a stain. (Fig. ()8.) Alter- 

 nating with the elastic tissue are smooth muscle and collagenous 

 fibers. The media appears to be very much thicker than the 

 adventitia, which is composed of fibroelastic tissue with few, if 

 any, smooth muscle cells. At the origin of the aorta and pul- 



