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HANDBOOK OF PHYSIOLOGY. 



cells (Fig. 101, ), 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. 



FIG. 101. Two blood-vessels from a frog's mesentery, injected with nitrate of silver, showingthe 

 outlines of the endothelial cells, a. Artery, The endpthelial cells are long and narrow; the trans- 

 verse markings indicate the muscular coat. t.a. Tunica adventitia. v. Vein, showing the shorter 

 and wider endothelial cells with which it is lined; c, c. two capillaries entering the vein. (Scho- 

 field.) 



tr.n. 



Ln. 



FIG 102. -Blood-vessels from mesocolon of rabbit, a. Artery, with two branches, showing tr. 

 n. nuclei of transverse muscular fibres; Ln. nuclei of endothelial lining; t.a. tunica adventitia. v. 

 Vein Here the transverse nuclei are more oval than those of the artery. The vein receives a small 

 branch at the lower end of the drawing; it is distinguished from the artery among other things by 

 its straighter course and larger calibre, c. Capillary, showing nuclei of endothelial cells, x 300, 

 (Schofield.) 



