118 THE CIRCULATION. 



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 superficially, while the 

 arteries are beneath the muscles, well protected from pres- 

 sure ; 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 the circulation, namely, the capil- 

 lary vessels, so called (from the Latin word capillarix, 

 hair-like), because of their extreme fineness. They are, 

 however, smaller than any hair, having a diameter of about 

 Tntair f an inch, and can only be observed by the use of the 

 microscope. These vessels may be regarded as the connect- 

 ing 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 work. The capillaries were not discovered until 

 1661, a short time after the invention of the microscope. 



40. The circulation of the blood, as seen under the 

 microscope, in the transparent web of a frog's foot, is a 

 spectacle of rare beauty, possessing more than ordinary 

 interest, when we consider that something very similar 

 is taking place in our own bodies, on a most magnificent, 

 scale. It is like opening a secret page in the history of 

 our own frames. We there see distinctly the three classes 

 of vessels with their moving contents; first, the artery, 



39. Capillaries? How regarded ? Harvey? 



40. The circulation of the blood in the web of a frog's foot? Describe it. 

 How general is the existence of the tissues t 



