184 THE BLOOD VASCULAR SYSTEM 



Comparison of Large and Small Arteries. The larger arteries are 

 typically elastic, the smaller typically muscular. In the larger vessels 

 the elastic tissue forms about one-half of the entire wall; toward the 

 smaller arteries this tissue progressively diminishes until, in the ar- 

 terioles, it is limited to an incomplete internal elastic membrane, the 

 homologue of the complete elastic coat or fenestrated coat of Henle, 

 which is found only in larger vessels. 



The smooth muscle, on the other hand, increases in relative amount 

 from the larger to the smaller arteries. While in the largest vessels it 

 forms not more than one-third, in the arterioles it represents about three- 

 fourths of the arterial wall. 



In the largest arteries the adventitia is relatively very thin. That 

 of the medium-sized vessels is much thicker, and the ratio of connective 

 tissue as found in the wall of these vessels remains fairly constant down 

 to the arterioles. In the wall of the precapillary arteries connective 

 tissue is very scanty. 



CAPILLARIES 



The capillaries are minute tubes, 5 to 13 p in diameter, which, in 

 nearly all the tissues of the body, connect the arteries with the veins. 

 Their wall is formed by a layer of endothelial cells which on the one 

 hand is continuous with the endothelial lining of the arteries, on the 

 other hand with that of the veins. 



As a rule there are neither muscle fibers nor connective tissue in 

 the wall of the true capillaries ; occasionally, however, very fine isolated 

 circumferential elastic fibers encircle the endothelial tube. In the 

 minute arterioles and venules, which are about to terminate in or take 

 origin from the true capillaries and which have been described as pre- 

 capillary arterioles and venules, a very thin layer of muscle fibers or of 

 connective tissue is added to the endothelial wall of the capillary. On 

 the arterial side the muscle is the first tissue to be thus added, on the 

 venous side the fibrous connective tissue is the first to appear. 



The endothelium of the capillary wall consists of flattened plate- 

 like cells which are joined edge to edge by cement substance. These 

 cells are somewhat elongated in the axis of the vessel, the shape of the 

 cell, as in the arteries and veins, depending upon the size of the vessel, 

 the smaller the vessel the more elongated its endothelial cells. The 

 margins of these cells are extremely irregular, hence they present a wavy 

 or serrated outline. 



