IV LABOURED RESPIRATION 145 



raising of the ribs now comes into action ; air is driven 

 out of the lungs and the diaphragm rises to its former 

 position (Fig. 48, B), being partly also pushed up by the 

 abdominal viscera which were pushed down when the 

 diaphragm contracted. At the same time gravity acting 

 on the ribs tends to lower them, and this is assisted by the 

 elastic recoil of the lungs and of the tissues of the chest 

 wall which had been put on the stretch during inspiration, 

 and possibly also by the contraction of the internal inter- 

 costal muscles. By these means air is driven out of 

 the lungs, the forcing out of the air constituting an 

 expiration, which taken together with an inspiration 

 makes up respiration. 



Thus it appears that we may have either diaphragmatic 

 resjnration, or costal resviration. As a general rule, how- 

 ever, the two forms of respiration coincide and aid one 

 another, the contraction of the diaphragm taking place at 

 the same time with that of the external intercostals, and 

 its relaxation with their relaxation. 



In ordinary quiet respiration, inspiration is an active 

 process depending on the contraction of muscles ; ex- 

 piration, on the other hand, is rather due to a passive 

 recoil of elastic structures which had been previously put 

 on the stretch. But at times, as when taking violent 

 exercise, the respiration becomes more forcible or, as it is 

 called, " laboured." In this case many accessory muscles 

 come into play to assist during inspiration in raising the 

 ribs and stei'num ; being chiefly muscles stretched between 

 the ribs and parts of the vertebral column — above them 

 at the back, and between the neck and the sternum in 

 front. At the same time expiration, from being passive 

 now also becomes an active process, chiefly by the con- 

 traction of certain muscles which connect the ribs and 

 breast-bone with the pelvis, and form the front and side 

 walls of the abdomen, the abdominal muscles. They 

 assist expiration in two ways : first, directly, by pulling 

 down the ribs ; and next, indirectly, by pressing the 



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