MECHANICS OF RESPIRATORY MOVEMENTS 1043 



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 in all three diameters of this cavity. The 

 levatores costarum, which are often included in inspiratory movements, are 

 so inserted into the ribs as to be unable to influence their movements. 

 They are concerned, not in respiration, but in lateral movements of the 

 spine. 



These muscles are the only ones normally engaged in carrying out in- 

 spiration. When, in consequence of muscular exertions or from any other 

 cause, the inspiratory efforts become more forcible, a large number of 

 accessory muscles are brought into play. These are : 



The scaleni, 



Sterno-mastoid, 



Trapezius, 



Pectoral muscles, 



Rhomboids, and 



The serratus anticus. 



Normal expiration is chiefly effected passively. When the inspiratory 

 muscles cease to contract, the lungs, which were stretched by the previous 

 inspiration, contract by virtue of the elastic tissue they contain, and the 

 thorax itself sinks by its own weight, and by the elastic reaction of the 

 stretched costal cartilages. 



It must be remembered, however, that in a position of rest the elasticity 

 of the thorax is opposed to the elasticity of the lungs. Elasticity of the 

 chest wall would therefore tend to produce inspiration. This factor would 

 tend to make inspiration easier at its onset, but would also present an 

 impediment to the carrying out of expiration, so that towards the end of this 

 act there is need for the active co-operation of muscular contractions. It 

 seems possible that more or less muscular activity of the expiratory muscles 

 is alternated with that of the inspiratory muscles. In fact Sherrington's 

 results on the co-ordination of muscular movements would tend to make us 

 assume inhibition of the tone, e.g. of the abdominal muscles, during inspira- 

 tion, and active augmentation of their tone during expiration. Where the 

 tone of the muscles is entirely lost, e.g. in the condition of viscero-ptosis, 

 it has been observed that the diaphragm is thrown out, of action, breathing 

 being chiefly carried out by an elevation of the upper part of the thorax. 

 Probably under normal circumstances the internal intercostal muscles 

 also contract with each expiration. 



