AKTUKItiS 



177 



named arieries which are to be found in nearly all of the organs and 

 tissues of the body. 



Medium-sized Arteries. A medium-sized artery will be first de- 

 scribed, as presenting the typical arterial structure. Such a vessel con- 

 sists of three coats: 



1. The internal coat tunica intima, or interna. 



2. The middle coat tunica media. 



3. The external coat tunica adventitia, or externa. 



The internal coat, tunica intima, presents three layers, the innermost 

 being the layer of endothelial cells, the outermost a layer of elastic tissue, 

 the fenestrated coat of Henle, or internal elastic membrane ; between 

 these is a delicate fibrous 

 membrane or tunica pro- 

 pria, which constitutes the 

 middle layer. This layer is 

 regarded by some as the 

 product of the endothelium. 



The endothelium com- 

 prises only a single layer of 

 flattened or squamous cells, 

 placed edge to edge to form 

 a continuous membrane of 

 simple pavement epithelium. 

 These cells are irregularly 

 polygonal in outline, with 

 serrated margins, and are 

 somewhat elongated in the 

 direction of the axis of 

 the vessel. They are loosely 

 attached to the elastic 



membrane by the middle layer of fine fibrillar connective tissue, in whose 

 ground substance small branching connective tissue cells are found. The 

 thickness of this connective tissue layer varies proportionately to the 

 size of the vessel. In the largest arteries it increases in amount also 

 with age, becoming especially well developed in the aorta. In the smaller 

 arteries and in certain of the larger, e.g., external iliacs, and the main 

 branches of the abdominal aorta, it is so scant as to be essential- 

 ly lacking. The thickening of the intima in the aorta coincident 

 with increasing age is commonly interpreted as a compensatory 

 mechanism necessitated by the increasing diameter of the vessel 



FIG. 196. A SMALL ARTERY FROM THE CON- 

 NECTIVE TISSUE OF THE ANTERIOR CERVICAL 

 REGION OF MAN. 



a, tunica adventitia; i, tunica intima; m, tunica 

 media; n, a small non-medullated nerve trunk; 

 v, a minute venule. Hematein and eosin. X 370. 



