AKTUKLKS 17? 



named arteries 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- ; . ^ . : -^^V-vsr=~vC *' 



pria, which constitutes the . ;.,,: .-:-'/" '''-',:'-[' ^:^^3\\ *\VA', 



middle layer. This layer is '"- .;/;":-/> ':' : .-.^ ^^j^X\\v. 



rrgardcd by some as I he . "'^iV'c; 



product of the cndothelium. t '^',','i'i & 



The endothelium com- 

 prises only a single layer of <)3 ' 

 flattened or squamous cells. . v%*'\\ 

 placed edge to edge 1o form , ' '^'\i 

 a continuous membrane of < " -HxC v '';'':;'''"' -/ 

 simple pavement epithelium. ' " %j%j?&^ n 

 These cells are irregularly 



polygonal in outline, with FIG. 196. A SMALL ARTERY FROM THE CON- 



serrated margins, .,! are ? -^ E *~ C 



somewhat elongated in the 



f . a, tunica ad ventitia; x, tunica mtima; m, tunica 



direction ot the axis of media; n> a gmall non -medullated nerve trunk; 

 the vessel. They are loosely v, a minute venule. Hematein and eosin. X 370. 

 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 



