172 THE CIRCULATION 



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 hinder the circulation of the blood, 

 are to be avoided as injurious to the health. 



42. 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 capittaris, hair-like), because of 

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

 hair, having a diameter of about y-^-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. 



43. The flow of blood through the veins is dependent upon 

 three different forces. First, the pressure from the capillary 

 circulation, second, the contraction of the muscles, and third, 

 the respiration. The pressure from the capillaries is the re- 

 maining impulse originally derived from the heart, and as the 

 capillary veins increase in size, from the capillaries toward the 

 larger veins, the flow is naturally facilitated. The action of 

 the muscles is exerted through their contraction. The veins 

 in the arms and legs, as well as in most parts of the body, lie 

 between the muscles, so that each contraction of the latter 

 squeezes the blood out of the veins and forward toward the 

 heart, the valves in them preventing any flow the other way. 

 The force exerted through the respiration is during the act of 

 inspiration. At this time the chest expands and enlarges in 

 all directions, allowing the veins of the thorax to fill more 

 readily, and thus making room for the blood in the veins of 

 other parts of the body. (Head Note 13.) 



42. Capillaries f How regarded ? Harvey ? 



43. On what three forces ie the flow of blood dependent ? Describe them. 



