In the diagram, Figs. 60, 61, D represents the diaphragm, which resembles an inverted bowl. Above it are the heart and lungs, marked H and L, and these are held up by blood-vessels and other supports above them. In this position of the diaphragm the air-vessels of the lungs are only partially filled with air, and there are two modes of increasing this supply. One is by _chest_ breathing, when the ribs are lifted upward and outward, making a vacuum in the air-vessels of the lungs. At the same time, the diaphragm is flattened by this expansion of the chest, as shown by the dotted lines. Then the air presses in through the nose and windpipe and fills the air-vessels, giving up its oxygen to the blood, and receiving carbonic acid and water, which are expired when the ribs and diaphragm return to their natural position. [Illustration: Fig. 60.] [Illustration: Fig. 61.] The other mode of filling the lungs is by _abdominal_ breathing, as illustrated by Fig. 61. At D is a side view of the diaphragm in its natural position, and the dotted lines show its position when it is contracted and thus flattened. When the diaphragm contracts or flattens, a vacant space is left above it, and then the air rushes in to fill the vacuum, as it does when the ribs are raised. This flattening of the diaphragm presses all the viscera beneath it downward, and thus causes the abdomen to swell outward, as is represented by the dotted lines at A. Then, when the diaphragm returns to its natural state, a vacant space is made beneath it, and in consequence the viscera below rises to fill the vacuum, owing to the pressure of the atmosphere around the body; for it is said that “nature abhors a vacuum,” by which is expressed a law of pneumatics in a popular adage. This law is, that when a vacuum is made in either air or water, the surrounding fluid presses from all sides, and from the bottom as strongly as from above. And thus, when a vacuum is made by the raising of the diaphragm, there is a pressure on all sides of the body, forcing the intestines upward to fill the vacuum thus made. This enables us to explain that most curious and wonderful mode by which the upper viscera are prevented from sinking on to the lower, as secured chiefly by abdominal breathing. The _pelvis_ is the bony basin supporting the spine, to which the bones of the legs are fastened.