In principle the structure of a gill corresponds with that of a lung; since we are particularly interested in the working of our own bodies we shall content ourselves with describing only a lung. We saw in the chapter on the Circulation that the pulmonary artery which leads away from the right side of the heart breaks up into a system of capillaries. These capillaries are thousands in number and they are spread out over the whole lung surface. The lung itself consists of a hollow bag with very thin and very elastic walls connecting with the throat by means of the windpipe. In reality the bag is double, for the windpipe splits at its lower end into two tubes, known as the chief bronchial tubes, and these subdivide repeatedly until their fine terminals end in the elastic lung sacs themselves. We spoke of the lung as a bag; in reality it is a system of thousands upon thousands of separate tiny bags. The structure is comparable to that of a bunch of grapes, the stem representing the chief bronchial tube, the smaller stems the subdivisions and the individual grapes the lung sacs proper. The advantage of this arrangement is, of course, in the very large surface which it gives; every one of the individual lung sacs has its wall filled with capillaries and there are so many of the tiny individual sacs that the total surface over which the blood is spread is measured in hundreds of square feet. We cannot imagine any other arrangement by which so large a surface of exposure could be packed away into a cavity the size of the human chest.