MECHANICS OF THE RESPIRATORY MOVEMENTS 1091 



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 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 respira- 

 tory 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 shorb 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 thorax 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. 



The movements of the thorax are effected by means of muscles. Inspira- 

 tion is performed by the following muscles : 



The diaphragm, which is the most important, and almost suffices alone 

 to carry out quiet respiration. 



The external intercostal muscles, which shorten and so raise the ribs. 



The serratus posticus superior. 



It is probable that an important part is played even under normal 

 circumstances in the respiratory movements by the extension of the spinal 

 column. This movement, which is specially marked at the upper part of 

 the thorax, causes an increase on all three diameters of this cavity. The 

 levatores costarum, which are often included in the inspiratory muscles, are 



