RESPIRATORY MOVEMENTS 111 



and laterally to the sternum and the lower ribs respectively. 

 In its concavity he the hver and stomach. In any down- 

 ward movement of the diaphragm the abdominal viscera 

 are depressed, and being incompressible must be accom- 

 modated by protrusion of the abdomen. The diaphragm 

 and abdominal muscles are therefore antagonistic. If the 

 abdominal wall is fixed, then the dome of the diaphragm 

 cannot be depressed. Under such circumstances contrac- 

 tion of the diaphragm will have the effect of drawing the 

 anterior and lateral attachment of the muscle up towards 

 the dome. The attachment of the diaphragm to the lower 

 costal cartilages draws the antero-lateral part of the lower 

 ribs outwards and forwards, the subcostal angle being 

 increased. Thus there is brought about an increase in the 

 capacity of this part of the chest. Since the abdominal 

 contents offer a certain resistance to the descent of the 

 diaphragm, contraction of this muscle results not merely 

 in a depression of its dome, but also in an elevation of 

 its circumferential attachment. That is to say that in 

 inspiration neither the dome nor the circumference is 

 fixed, but the former moves downwards and the latter 

 upwards. 



Movement of the ribs is due mainly to the action of the 

 intercostal and interchondral muscles, but owing to the 

 variation in the size, shape and disposition of the different 

 ribs no general rule can be laid down as to the manner in 

 which these muscles act. 



In the lower or cyhndrical part of the chest the external 

 intercostals, running as they do downwards and forwards, 

 reduce the obhquity of the ribs and in this way assist 

 inspiration. The internal intercostals, on the other hand, 

 since they are directed downwards and backwards, cause 

 expiration by increasing the obhquity of the ribs. This 

 will be clear from the accompanying figures. A and B 

 represent spine and sternum respectively, C and D two 

 ribs. In the upper figure xy represents an external inter- 

 costal muscle. Imagine the four corners of the parallelo- 



