MECHANICS OF RESPIRATORY MOVEMENTS 1165 



The upper surface of the central tendon is united to the pericardium. 

 This part, during expiration, is the deepest part of the middle portion 

 of the diaphragm. Towards the back of the pericardial attachment 

 the central tendon is pierced for the passage of the inferior vena cava. 

 In expiration the lateral muscular zone of the diaphragm lies in con- 

 tact with the lower part of the thoracic wall. During inspiration the 

 muscle fibres contract and draw the central tendon downwards, so 

 that the lower surface of the lungs descends. The enlargement of the 

 lungs at the lower part of the thorax is aided by the abduction of the 

 floating ribs, produced by the contraction of the quadratus lumborum 

 and deep costal muscles. In this contraction the diaphragm presses 

 on the contents of the abdomen, so that the 

 abdomen swells up with each inspiratory move- 

 ment. The middle of the central tendon, where 

 the heart lies, moves less than the two domes, 

 and the part where the vena cava passes 

 through the tendon is practically stationary 

 during normal respiration. In deep inspiration, 

 however, both this part as well as the rest of 

 the pericardial attachment is forcibly depressed 

 towards the abdomen. In quiet breathing, 

 when observed by the Rontgen rays, the mean FIG. 487. Diagram show- 

 descent of the right dome in inspiration has ^ movements of dia- 

 phragm in respiration. 



been found to be about 12-5 mm., and of the ^inspiratory position; 

 left dome 12 mm. We may say, roughly, that e, expiratory position, 

 the average descent of the diaphragm during 



normal respiration is about half an inch. The viscera and the 

 intra-abdominal pressure play an important part in determining 

 the movement of the diaphragm, and especially in preserving the 

 abduction of the lower ribs and so furnishing a fixed point for the 

 muscular fibres of the diaphragm. If the contents of the abdomen 

 are removed from a living animal the ribs are drawn inwards every time 

 the diaphragm contracts. In children with weak chest walls and with 

 respiratory obstruction we may often see a depression round the lower 

 part of the chest corresponding to the lower border of the lungs. It 

 corresponds to the line at which the diaphragm leaves the chest wall, 

 so that the distending force of the abdominal pressure on the bony walls 

 of the thorax abruptly gives place to the pull of the distended lung. 

 The contraction of the diaphragm lasts four to eight times longer 

 than a simple contraction or muscle-twitch. It may be regarded 

 therefore as a short tetanus. 



The enlargement in the other diameters is effected by an elevation 

 of the ribs. Each pair of corresponding ribs, which are articulated 

 behind with the spinal column and in front with the sternum, forms 



