CHAPTER XVII. 



THE BLOOD VESSELS. 



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). (Schft- 

 fer.) (a) Endothelium lining the vessel, appearing thicker than natural from the con- 

 traction of the outer coats; (fe) the elastic layer of the intinia; (c) middle coat, com- 

 posed of muscle fibres and elastic tissue; (d) outer coat, consisting 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. 



Arterial twigs of considerable size here and there form con- 

 nections with those of a neighboring trunk (anastomoses), but 



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