CHAPTER XXXII 

 THE MECHANICS OF BREATHING 



To facilitate gaseous interchange, the process of breathing or 

 ventilation of the lungs takes place. In the act of inspiration in 

 mammals the chest is expanded, air is drawn into the lungs ; in 

 the act of expiration the chest and lung capacity is diminished, and 

 air forced out from the lungs. 



Anatomical Considerations. To understand properly the move- 

 ments concerned in the processes of inspiration and expiration, certain 

 anatomical details in regard to the bony framework and the muscula- 

 ture of the thorax have to be considered. The varying extensibility 

 of different parts of the lung has also to be borne in mind. 



In the inspiratory movement, the thorax is expanded in three 

 dimensions. In regard to the exact manner in which these move- 

 ments take place there is still some uncertainty. The chief move- 

 ments, however, may be grouped as 



1. The movements of the diaphragm. 



2. The movements of the ribs and rib cartilages. 



Breathing of the abdominal type, such as occurs in man, is 

 chiefly diaphragmatic; breathing of the thoracic type, such as takes 

 place in corsetted women, is mainly costal. Normally, breathing is 

 a combination of the two types sometimes one, sometimes the other 

 prevails. 



The Action and Movements of the Diaphragm. Separating the 

 thorax from the abdomen, the diaphragm, in its resting position, reaches 

 up to about the fifth intercostal space. The fleshy part of the muscle 

 lies close to the ribs, the central dome-like part being mainly of a 

 tendinous nature. There has been considerable speculation as to the 

 exact nature of the movements performed by the diaphragm. The 

 view commonly expressed is that the muscle, by its contraction, in 

 association with muscles which fix the thrcax, opens up the angle 

 which it forms with the thoracic wall, thereby enabling the lung 

 to expand in a downward direction. In such a movement the 

 central tendinous portion is supposed not to participate. After 

 contraction, it is assumed that the diaphragm passively returns to 

 the position of rest. 



Recent study of the diaphragmatic movement by means of the 

 X rays has shown, however, that there is a forward downward move- 



23 A 



