38. This simple experiment proves that the true direction of the venous blood is toward the heart. That the color {118} 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 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. [Sidenote: 39. Capillaries? How regarded? Harvey?] 39. THE CAPILLARIES.--A third set of vessels completes the list of the organs of the 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 1/3000 of an inch, and can only be observed by the use of the microscope. These vessels may be regarded as 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 work. The capillaries were not discovered until 1661, a short time after the invention of the microscope. [Sidenote: 40. The circulation of the blood in the web of a frog's foot? Describe it. How general is the existence of the tissues?]