CIRCULATORY SYSTEM. 



109 



epithelial cells secreting a serous fluid that acts as a 

 lubricant. 



ARTERIES AND VEINS. 



Arteries. The arteries convey blood from the 

 heart to the capillaries, and vary in size from the 

 aorta, the largest, down to minute structures of mi- 

 croscopic caliber. The walls of these vessels are 

 composed of three layers : tunica intima, media, and 

 adventitia. 



i . Tunica intima is the internal coat and is a very 





Fig. 76. Cross section of small artery and vein; A, artery; V t vein. 



thin, smooth, glassy membrane, often difficult to 

 demonstrate in sections. This is again divided 

 into three layers, the innermost being a layer of 

 pavement endothelial cells, outside of which we find 

 a delicate fibrous connective-tissue fabric, the sub- 

 endothelium, and outside of this again a layer of elas- 

 tic fibers called the fenestrated membrane of Henle. 

 The endothelial layer is made up of a single layer of 

 flattened cells, held together by a cement substance 

 and analogous to the endothelium of the peritoneum 



