THE CIRCULATION OF THE BLOOD 



versely to the long axis of the artery, figure 143, w, 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 

 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 thefenestrated membrane of Henle. 

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



FIG. 143. 



FIG. 144. 



FIG. 145. 



FIG. 143. Minute Artery Viewed in Longitudinal Section, e, Nucleated endothelial 

 membrane, with faint nuclei in lumen, looked at from above; i, thin elastic tunica intima; 

 m, muscular coat or tunica media; a, tunica adventitia. (Klein and Noble Smith.) 



FIG. 144. Transverse Section through a Large Branch of the Inferior Mesenteric 

 Artery of a Pig. e, Endothelial membrane; i, tunica elastica interna, no subendothelial 

 layer is seen; m, muscular tunica media, containing only a few wavy elastic fibers; e, c, 

 tunica elastica externa, dividing the media from the connective-tissue adventitia, a. Mag- 

 nification, 350 diameters. (Klein and Noble Smith.) 



FIG. 145. Muscular Fiber Cells from Human Arteries. Magnified 350 diameters, 

 a, Nucleus; B, a fiber cell treated with acetic acid. (Kolliker.) 



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 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. 



Many of the arteries are accompanied by a plexus of vaso-motor nerves. 

 In the smaller arteries these nerves consist of few fibers that form a delicate 

 network over the walls of the vessels. Many fibers appear to end in the 

 muscle cells of the arterioles in the proximity of the nuclei. 



The Capillaries. In all vascular textures, except some parts of the 

 corpora cavernosa of the penis, of the uterine placenta, and of the spleen, 



