MECHANICS OF THE RESPIRATORY MOVEMENTS 1093 



muscles, and from a consideration of Fig. 500 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, musculi intercartilaginei, 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 



FIG. 499. 



FIG. 500. 



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 

 products, or the liver, a dull sound is obtained. It is easy to show that the 

 resonant area of the chest increases with each inspiration. The apices of 

 the lungs extend about one inch above the clavicle anteriorly and behind 

 reach as high as the seventh spinous process. During moderate expiration 

 the lower margin of the lungs extends in front from the upper border of the 

 sixth rib at its insertion to the sternum, and runs obliquely downwards to 

 the level of the tenth rib at the back of the chest. During the deepest 

 inspiration the lungs descend in front to the seventh intercostal space 

 and behind to the eleventh rib, while during deepest possible expiration the 



