150 



THE CIRCULATION OF THE BLOOD 



145, m, is composed of both muscular and elastic fibers with a certain pro- 

 portion of areolar tissue. In the larger arteries, figure 145, its thickness is 

 comparatively as well as absolutely much greater than in the small arteries, 

 constituting, as it does, the greater part of the arterial wall. The muscular 

 fibers are unstriped, figure 146, and are arranged, for the most part, trans- 

 versely to the long axis of the artery, figure 144, m, while the elastic element, 

 taking also a transverse direction, is disposed in the form of closely inter- 

 woven and branching fibers intersecting in all parts the layers of muscular 

 fiber. In arteries of various size there is a difference in the proportion of 



FIG. 147. Vein and Capillaries. Silver-nitrate and hematoxylin stain, to show outlines 

 of endothelial cells and their nuclei. (Bailey.) 



the muscular and elastic element, elastic tissue preponderating in the largest 

 arteries and unstriped muscle in those of medium and small size. The 

 arteries are quite elastic in both large and small vessels. The internal coat 

 is formed by a layer of elastic tissue, called the jenestrated membrane of Henle. 

 It is peculiar in its tendency to curl up when peeled off from the artery, and 



FIG. 148. Network of Capillary Vessels of the Air Cells of the Horse's Lung Magnified, a, a, 

 Capillaries proceeding from b, b, terminal branches of the pulmonary artery. (Frey.) 



in the perforated and streaked appearance which it presents under the micro- 

 scope. The inner surface of the artery is lined with a delicate layer of elon- 

 gated endothelial cells, figure 145, e, which make it smooth and polished and 

 furnish a nearly impermeable surface along which the blood may flow with 

 the smallest possible amount of resistance from friction. 



