164 THE CIRCULATION 



disposed in such a way that they open only to receive the 

 blood flowing toward the heart, and close against a current in 

 the opposite direction. Their position in the veins on the back 

 of the hand may be readily observed, if we first obstruct the 

 return of blood by a cord tied around the forearm or wrist. 

 In a few moments the veins will appear swollen, and upon 

 them will be seen certain prominences, about an inch apart. 

 These latter indicate the location of the valves, or, rather, they 

 show that the vessels in front of the valves are distended by 

 the blood, which cannot force a passage back through them. 



38. This simple experiment proves that the true direction 

 of the venous blood is toward the heart. That the color of the 

 blood is dark will be evident, if we compare the hand thus 

 bound by a cord with the hand not so bound. It also proves 

 that the veins lie near the surface, while the arteries are 

 beneath the muscles, well protected from pressure; and that 

 free communication exists from one vein to another. If now 

 we test the temperature of the constricted member by means 

 of a thermometer, we will find that it is colder than natural, 

 although the amount of blood is larger than usual. From this 

 fact we infer, that whatever impedes the venous circulation 

 tends to diminish vitality; and hence, articles of clothing or 

 constrained postures, that confine the body or limbs, and hinder 

 the circulation of the blood, are to be avoided as injurious to 

 the health. 



39. The Capillaries. A third set of vessels completes the 

 list of the organs of circulation, namely, the capillary vessels, 

 so called (from the Latin word capillaris, hair-like), because of 

 their extreme fineness. They are, however, smaller than any 

 hair, having a diameter of about -g-^ of an inch, and can only 

 be observed by the use of the microscope. These vessels are 

 the connecting link between the last of the arteries and the 

 first of the veins. The existence of these vessels was unknown 

 to Harvey, and was the one step wanting to complete his great 

 discovery. The capillaries were not discovered until 1661, a 

 short time after the invention of the microscope. 



88. What will be proved by the experiment ? What inference is drawn 1 



89. Capillaries? How regarded? Harvey? 



