210 ANATOMY OF THE DOMESTIC FOWL 



with the venules, or terminal veins. They are only from 6 to 14 

 microns in diameter. Their walls consist of a single layer of 

 endothelial cells, which are somewhat elongated in the long axis 

 of the vessels. Their edges are serrated, and are united by a small 

 amount of intercellular cement-like substance. Capillaries branch 

 without diminution in caliber, and these branches anastomose to 

 form capillary networks, the meshes of which differ in size and shape 

 in different tissues and organs. The largest meshed networks occur 

 in the serous membranes and hi the muscles; and the smallest occur 

 in the glands, such as the liver. 



The walls of the arteries are thick and stand open when empty, 

 owing to the elastic tissue contained in their walls, while the walls 

 of the veins collapse when empty, owing to their containing a smaller 

 amount of elastic tissue. The arterial wall is provided with three 

 coats: tunica intima } or inner coat; tunica media, or median coat; 

 and tunica adventitia, or outer coat. 



The tunica intima consists of a single layer of endothelial cells, 

 continuous with and similar to that forming the walls of the capil- 

 laries. In passing from the capillaries to the arterioles, there is first 

 a thin coat, or sheath-like layer, of connective tissue around the 

 outside of the endothelial tubes. Further along, isolated smooth 

 muscle cells arranged in a circular manner occur between the endo- 

 thelial layer and the layer of connective tissue, this structure 

 forming vessels called precapillary arteries. Further along still, 

 the muscle cells form a complete layer; in this section the vessels 

 are called arterioles and are made up of three coats: the inner endo- 

 thelial, the middle muscular, and the outer fibrous. 



In arteries of medium size the intima consists of the endothelial 

 layer, a layer of delicate white and elastic fibers, connective-tissue 

 cells, and the membrana elastica interna, or an outer layer, the 

 elastic layer, of the intima. 



The media consists of a thick coat of circularly arranged smooth 

 muscle cells, its thickness depending largely upon the size of the 

 vessels. There is also a small amount of fibrillary connective tissue, 

 which supports the muscle cells. Elastic tissue is present in the 

 media, the amount depending on the size of the vessel, the larger 

 the vessel the more elastic tissue there is present. In the large 

 arteries coarse elastic fibers intermingle with the finer ones. When 

 much elastic tissue is present the muscle cells are separated into 

 more or less well-defined groups. 



