1042 PHYSIOLOGY 



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 movement. 

 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 ob- 

 served by the Eontgen rays, the mean descent of the right dome in inspiration 

 has been found to be about 12-5 mm., and of the left dome 12 mm. We may 

 say, roughly, that 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 dia- 

 phragm, 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 con- 

 traction 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 a ring directed 

 obliquely from behind downwards and forwards. With each inspiratory 

 movement the ribs are raised, the obliquity becomes less, and the horizontal 

 distance between sternum and spinal column is therefore increased. More- 

 over the ribs from the first to the seventh increase in length from above 

 downwards, so that when they are raised, the sixth rib, for instance, occupies 

 the situation previously taken by the fifth, and the transverse diameters of 

 the thorax at this height are increased. With each inspiration there is a 

 rotation of the ribs. In the expiratory condition they are so situated that 

 their outer surfaces are directed not only outwards but also downwards. 

 As they are raised by the inspiratory movements, they rotate on an axis 

 directed through the fore and hind ends of the rib, so that their outer 

 surfaces are turned directly outwards. In this way a certain enlargement 

 of the thoracic cavity is produced. As the thorax is raised there is always 

 some stretching of the rib- cartilages. 



In expiration the processes are reversed, and the cavity of the thorax 

 is diminished in all three dimensions. 







