1168 PHYSIOLOGY 



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 niedias- 

 tinal 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 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 

 lower margins of the lungs are elevated almost as much as they descend 

 during inspiration. In the front of the chest a triangular space can 

 be always marked out over the heart where the note obtained on per- 

 cussion is dull. This space .is bounded on the right by the left border 

 of the sternum and extends out as far as the cardiac apex, being bounded 

 above by the fourth costo-sternal articulation and below by the sixth 

 costal cartilage. 



BREATH SOUNDS. If the ear be applied to the chest wall, either 

 directly or through the medium of a stethoscope, each inspiration is 



