Inspiration requires a good deal of muscular effort. When the diaphragm contracts and flattens its dome it has to push down the abdominal viscera on its under side, and to press out the front wall of the abdomen to make room for them. The ribs and breast-bone have also to be pulled up out of their natural position of equilib rium. In ordinary expiration, on the contrary, but little if any muscular effort is required.

"What is the general direction of the ribs? How is it altered during inspiration? What is the consequence? Illustrate by reference to a diagram.

Why does inspiration require muscular effort?

As soon as the muscles which have raised the ribs and sternum relax, these bones return to their-natural unconstrained position; and the elastic abdominal wall presses the abdominal viscera against the under side of the diaphragm and pushes that organ up again, as soon as its muscular fibres cease contracting. In this way the chest cavity is restored to its original capacity, and the air is sent out of the lungs rather by the elasticity of the parts which were stretched in inspiration, than by any special expiratory muscles.

When, however, an expiration is violent (when, for example, we try to empty our lungs of air as completely as possible, or during a fit of coughing) special expiratory muscles, which pull down the ribs and press up the diaphragm, are called into action.