AUXILIAEY MUSCLES OF INSPIRATION. 381 



horse, winch does not respire by the mouth, these movements 

 are as essential to life as are the respiratory movements of the 

 larynx. In man, as a rule, the nares undergo no movement 

 unless respiration be somewhat exaggerated. In very diffi- 

 cult respiration the mouth is opened at each inspiratory act. 

 We have not thought it necessary to treat of the action of 

 those muscles which serve to fix the head, neck, or shoulders 

 in dyspnoea. 



The division into muscles of ordinary inspiration, ordi- 

 nary auxiliaries, and extraordinary auxiliaries, must not -be 

 taken as absolute. In the male, in ordinary respiration, the 

 diaphragm, intercostals, and levatores costarum are the great 

 inspiratory muscles, and the action of the scaleni, with the 

 consequent elevation of the sternum, is commonly very slight, 

 or perhaps wanting. In the female, the movements of the 

 upper parts of the chest are very marked, and the scaleni, the 

 serratus posticus superior, and sometimes the sterno-mastoid, 

 are brought into action in ordinary respiration. In the vari- 

 ous types of respiration, the action of the muscles engaged in 

 ordinary respiration necessarily presents considerable varia- 

 tions. 



