391. The first step necessary to the ascent of the diaphragm is the relaxation of its muscular fibres. As soon as these fibres are in a state of relaxation, that is, when the organ has changed from an active to a completely passive state, the powerful muscles of the abdomen (fig. CXLVI. 6, 7, 8) contract, and push the abdominal viscera and the diaphragm with them upwards towards the cavity of the chest (fig. CXLV. 2); and thus, by the descent of the ribs and the ascent of the diaphragm, the capacity of the thorax is diminished in every direction, and the motion of expiration is completed. 392. Such is the mechanism by which the capacity of the thorax is alternately enlarged and diminished in the two alternate states of inspiration and expiration, and the mechanism thus adjusted works in the following mode. 393. Expiration succeeding to the state of inspiration, the ribs descend, the diaphragm ascends, the capacity of the thorax lessens, and the compressed lungs are forced within the smallest possible space. Then, inspiration, succeeding to the state of expiration, the ribs ascend and the diaphragm descends; the capacity of the thorax is enlarged, and the lungs freed from their pressure expand and fill the greater space obtained. In about a second and a half after the state of inspiration has been induced, that of expiration recommences; the motion of inspiration occupying about double the time of the motion of expiration, and these alternate conditions succeed each other in a regular and uniform course, day and night, during our sleeping and our waking hours to the end of life. 394. As long as the function is performed in a perfectly natural manner, a given number of these alternate movements takes place in a certain time, constituting what is termed the rhythm of the respiratory motions. These motions perfectly regular in number and time, are likewise, in the natural state of the function, performed only with a certain degree of energy; but they are variously modified at the command of the will; in obedience to numerous sensations and emotions; in the performance of a great variety of complex actions, and in different states of disease. These modifying circumstances may cause the action of inspiration to be more full and deep, and that of expiration to be more forcible and complete than natural; or they may cause both movements to be shorter and quicker than common: hence the distinction of respiration into ordinary and extraordinary.