The object of respiration is to purify the blood and render it fit for the various offices it performs in the animal economy. When the blood leaves the heart to be distributed through the body, it is of a very bright red colour, but as it proceeds in its course it gradually loses this and assumes a purple hue; and when, having completed its circulation through the body, it is returned to the heart again by the veins, it has entirely lost its former bright colour, and is then very dark, and, from the impurities it has acquired in its course, unfit for the purposes of life. To restore its former qualities it is necessary that it should be brought into contact with the atmosphere, and this takes place in the lungs. By the action of the muscles of the chest and abdomen, the interior of the chest is increased in size, an empty space is formed into which the air instantly descends by the mouth or nostrils, constituting what is termed the act of inspiration. At the same moment that the muscles of the chest increase its size and make room for the air to descend, at that very moment the heart contracts, or in popular language pulsates or beats, the effect of which is to force the dark-coloured (venous) blood from the portion of the heart in which it was contained, into the lungs. The lungs are composed almost entirely of an innumerable number of vesicles, or minute air-bladders, into which the air descends, as we have stated. These vesicles are covered with a network of extremely fine blood-vessels. When the heart pulsates, it fills these vessels with the dark-coloured blood; and as the air is capable of passing through both the coats of the vesicle and of the blood-vessels, it of course comes into direct contact with the blood, and a chemical change immediately takes place.