CHAPTER XVII. 



THE BLOODVESSELS. 



THE channels which carry the blood throughout the body form 

 a closed system of elastic tubes, which may be divided into three 

 varieties : 



1. Arteries. 



2. Capillaries. 



3. Veins. 



The arteries and veins serve merely to conduct the blood to and 

 from the capillaries, where the essential function of the blood, viz., 

 its chemical interchange with the tissues, is carried on. 



FIG. 124. 



Transverse Section of part of the Wall of the Posterior Tibial Artery 

 (man). (Schafer.) (a) Endothelium lining the vessel, appearing thicker 

 than normal from the contraction of (6) the elastic layer of the intima; (c) 

 middle coat, composed of muscle fibres and elastic tissue ; (d) outer coat, con- 

 sisting chiefly of white fibrous tissue. 



The arteries are those vessels which carry the blood from the 

 heart to the capillaries. The great trunk of the aorta, which 

 springs from the left ventricle, gives off a series of branches, 

 which in turn subdivide more and more freely in proportion to 

 their distance from the heart. Their mode of division is com- 

 monly dichotomous, but, from the larger trunk, branches of un- 

 equal and irregular size are frequently given off. 



