CAPILLARIES VEINS 175 



Note. The internal coat is the only one which is continuous throughout 

 the entire circulatory system. 



Surgical note. When a ligature is tied tightly around an artery the middle 

 coat may be felt to break down under the cord, while the external one remains 

 whole, owing to its toughness. 



The arteries give off branches which divide and subdivide until 

 the smallest ones can be seen only with the microscope they are 

 called arterioles. The arterioles lead to the vessels which are 

 smallest of all the capillaries. 



Capillaries. Vessels which receive blood from the arteries and 

 carry it to the veins. They exist in nearly every part of the body, 

 except cartilages, hair, nails, cuticle, and the cornea of the eye. 

 Their walls have only the internal coat, a single layer of cells 

 endothelium. It is through this thin wall that the work of 

 exchange is performed between the blood and the various tissues of 

 the body, nutritive material being taken from the blood and certain 

 waste substances being returned to it. To provide vessels for this 

 exchange is the function of the capillaries. They are most numer- 

 ous where most work is to be done, viz., in the lungs, skin, mucous 

 membranes, liver, kidneys and glands. 



Their average diameter is WFIO of an inch just enough to permit the easy 

 passage of the corpuscles. They are uniform in size, neither increasing nor 

 diminishing in caliber. 



Veins. The vessels which gather the blood from the capil- 

 laries and carry it to the heart; they are formed by the uniting of 

 capillaries. 



They are at first very small (called venules or venous radicles] but constantly 

 grow larger by uniting with each other, although they often branch and 

 reunite. 



Veins, like arteries, have three coats, but their middle coat is 

 neither so elastic nor so muscular, so that they are softer, and when 

 empty or cut, they collapse. The inner coat of the veins presents, 

 at intervals, semilunar folds, making pockets called valves, which 

 allow the blood to flow toward the heart, but prevent it from setting 

 backward freely. If the veins are very well rilled the location of 

 the valves may be recognized by an appearance of puffing out at 



