1092 PHYSIOLOGY 



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. 



Although the action of the intercostal muscles has been a subject of debate, physio- 

 logical experiments serve on the whole to confirm the view first put forward by Ham- 

 berger and based on a consideration of the direction of the fibres. The external inter- 

 costals pass from one rib to the next below downwards and forwards. Hence if a pair of 

 ribs be isolated from the rest of the chest wall, leaving the vertebral and costal attach- 

 ments intact, contraction of these muscles will cause a rise of both ribs. This result will 

 be evident from a consideration of Fig. 499, where ab is a fibre of the external intercostal 

 muscles, passing from the rib vs to be attached to the rib v's' at b. When ab contracts, 

 the tension it exerts on its two attachments can be resolved into two components ac acting 

 downwards and bd acting upwards, bd however acts at the end of the long lever bv', 

 whereas ac acts at the end of the short lever av. Hence the raising effect will overcome 

 the depressing effect, and both ribs will rise. 



The fibres of the internal intercostals run in the opposite direction to the external 



