EPITHELIAL AND ENDOTHELIAL TISSUE. 339 



numerous offshoots between the veins, and is in part freely sup- 

 plied with smooth muscles. 



Arteries and veins in specimens under the microscope may be 

 distinguished from each other by the fact that arteries almost 

 always are empty, or contain only a few blood-corpuscles, while 

 veins, as a rule, hold a large amount of blood. Besides, the arteries 

 are found to be contracted, their inner coat has a fluted, corrugated 

 outline, while the veins are more or less smooth in their inner 

 circumference. The wall of the artery is always broader than 

 that of the vein, owing to the presence of the muscle coat, which, 

 both in longitudinal and transverse sections, is well marked in the 

 former and but slightly marked in the latter. In transverse sec- 

 tions of veins, also, transverse sections of the longitudinal muscle- 

 fibers are often recognized. Another point of difference between 

 an artery and a vein, should the latter have a marked muscle 

 coat (see Fig. 145), is that the artery has a circular or slightly 

 oblong caliber, while the vein has an oblong, irregular, com- 

 pressed, or hour-glass shaped caliber. The adventitial coat, 

 likewise, is decidedly thicker in arteries than in veins, though 

 both may exhibit the nutrient capillary blood-vessels, the vasa 

 vasorum. 



(4) The Capillaries are composed of a single endothelial wall. 

 Their plexiform arrangement is best brought into view by the 

 injection of a stained, coagulating material. The density of the 

 reticulum greatly varies in different parts and organs of the 

 body. The closest reticulum is found in the lungs ; the densest, 

 and at the same time broadest, reticulum in the liver. In the latter 

 organ, the caliber of the vessels show marked variations in size 

 in comparatively small territories of the lobules ; this is evidently 

 due to a difference in the diameter of the liver epithelia in differ- 

 ent stages of their activity. In the kidneys, too, there are striking 

 differences in the calibers of the capillaries, some of which (the 

 so-called vasa recta) attain the size of veins. 



S. Strieker was the first to point out the fact that the wall of 

 the capillaries is endowed with vital properties, and especially 

 with that of contractility. Before him, the capillary walls were 

 considered to be only an elastic membrane. Nobody can doubt, 

 who has carefully observed capillaries in different situations, 

 that during life the endothelial wall exhibits marked changes, 

 according to its state of contraction, to the functional activity 

 of an organ, and to the age of the individual. The injection of a 

 colored material from without renders these differences, as a 



