1044 



PHYSIOLOGY 



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 Hamberger 

 and based on a consideration of the direction of the fibres. The external intercostals 

 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 attachments 

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

 evident from a consideration of Fig. 487, 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 



FIG. 488. 



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 

 muscles, and from a consideration of Fig. 488 it is evident that their effect will be to 

 depress any pair of ribs, thus acting as expiratory muscles. 



Owing to the fact that the costal cartilages make an angle with the bony ribs, the 

 fibres of prolongation of the internal intercostals, muscidi inter car tilaginei, have the 

 same relation to their attachments that the external intercostals have to the bony ribs. 

 Their action therefore must be to raise the cartilages and flatten out the angle between 

 the cartilaginous and bony ribs so that they must act with the external intercostals as 

 inspiratory muscles. 



In forced expiration a large number of muscles may take part such as 

 the serratus posticus inferior, and the muscles forming the wall of the 

 abdomen, i.e. the rectus, obliquus, and transversus abdominis muscles. 



As the lungs are distended with each inspiration their position changes 

 in relation to the thoracic wall. All parts are not equally distensible in the 

 normal position of the lungs. There are three areas which are in contact 

 with the nearly stationary parts of the thoracic wall and cannot therefore 

 be directly expanded. These are (1) the mediastinal surface in contact with 

 the pericardium and structures of the mediastinum ; (2) the dorsal surface in 

 contact with the spinal column and with the spinal segments of the ribs ; 

 (3) the apical surface lying in contact with the deep cervical fascia at the 

 root of the neck. The roots of the lungs move with inspiration somewhat 

 forwards and downwards. The front parts of the lungs move downwards 

 and inwards, so that their inner borders in front approach one another. 

 The extent and boundaries of the lungs can be easily ascertained in the 

 living subject by means of percussion. On tapping the finger laid on the 

 chest a sound is emitted which varies with the nature of the subjacent tissues. 

 If this is lung-tissue filled with air, a clear -resonant tone is obtained ; where 

 it is solid tissue, such as the heart, or a lung consolidated with inflammatory 



