STEUCTUEE OF BLOOD VESSELS 



189 



Structure of blood vessels. — (See Ex. XLYI.) Arteries. 

 A cross section of an artery (see Fig. 71) shows it to be 

 made up of three layers of 

 tissue. The outer coat is 

 composed of fibrous con- 

 nective tissue, which is 

 tough, and a strong pro- 

 tective layer. This coat 

 contains also some elastic 

 fibers that enable it to 

 expand imder the pulsing 

 of the blood. The middle 



layer consists of a mixture OTig. 71 — Ci-oss section of an artery; c, 



of Unstriped muscle cells abrons connective tissue outer sheatH; 



^ 7n, muscle cells and elastic fibers ; e, 



and elastic fibers, and is epithelial cells, with bands of tissue 



, , . , , , , , , , , separating them from the middle layer. 



thicker than the other two 



layers. The inner layer is made of flattened, epithelial 



cells, and between it and 

 the middle layer are a 

 varying number of bands 

 of membranous and elastic 

 tissue. 



Veins. The structure of 

 the veins is practically 

 identical with that of the 

 arteries (see Fig. 72) ex- 

 cept that the outer coat is 

 thickest and the two inner 

 layers are much less de- 

 veloped. Owing to this 

 difference they are much 



iess elastic, and readily collapse when empty. The 



Fig, 72 — Cross section of a vein ; c, outer 

 fibrous sheath ; m, middle muscle and 

 elastic tissue layer ; e, inner epithelial 

 cell layer. 



