CIRCULATION OF THE BLOOD. 



131 



mid streaked appearance which it presents under the microscope. Its 

 inner surface is lined with a delicate layer of endotfcelium, composed of 

 elongated cells (Fig. 112, a), 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. 



Immediately external to the endothelial lining of the artery is fine 

 connective tissue, sub-endothelial layer, with branched corpuscles. Thus 

 the internal coat consists of three parts, (a) an endothelial lining, (b) the 

 sub-endothelial layer, and (c) elastic layers. 



Vasa Vasorum. The walls of the arteries, with the possible excep- 

 tion of the endothelial lining and the layers of the internal coat immedi- 

 ately outside it, are not nourished by the blood which they convey, but 

 are, like other parts of the body, supplied with little arteries, ending in 



FIG. 111. 



FIG. 111. Transverse section of small artery from soft palate, e, endothelial lining, the nuclei 

 of the cells are shown; i, elastic tissue of the intima, which is a good deal folded; c. m. circular mus- 



. showing the 



Artery. The endothelial cells are long and narrow ; the trans- 

 vtM-st! markings indicate the muscular cout. 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. (Schofield.) 



capillaries and veins, which, branching throughout the external coat, 

 extend for some distance into the middle, but do not reach the internal 

 coat. These nutrient vessels are called vasa vasorum. 



Lymphatics of Arteries and Veins. Lymphatic spaces are pres- 

 ent in the coats of both arteries and veins; but in the tunica adventitia 

 or external coat of large vessels they form a distinct plexus of more or less 

 tubular vessels. In smaller vessels they appear as sinous spaces lined by 

 endothelium. Sometimes, as in the arteries of the omentum, mesentery, 

 and membranes of the brain, in the pulmonary, hepatic, and splenic 

 arteries, the spaces are continuous with vessels which distinctly ensheath 



