402 TEXT-BOOK OF PHYSIOLOGY 



inward were this not prevented by the outward pressure of the displaced 

 viscera. 



Coincidently with the descent of the diaphragm and the partial removal 

 of the pressure on the under surface of the lungs, the intra-pulmonic air ex- 

 pands. As it expands it distends the lungs in the vertical direction, causing 

 them to follow the diaphragm and to occupy the so-called complementary 

 pleural space. With the expansion of the intra-pulmonic air there is a fall 

 in its pressure below the atmospheric pressure, to be followed immediately 

 by an inflow of air until atmospheric pressure is again established. This 

 occurs at the end of the inspiration. 



The antero-posterior and transverse diameters are increased by the eleva- 

 tion and outward rotation of the ribs and an advance of the sternum, both 

 movements made possible by the construction and arrangement of the costo- 

 vertebral and costo-chondral and chondro-sternal articulations. The 

 construction of these articulations is such as to permit at the first a slight 

 elevation and depression of the head of the rib, and at the second a glid- 

 ing of the tubercle on the transverse process. The axis around which the 

 rib rotates practically coincides with the axis of the rib neck, which in the 

 upper part of the thorax is almost horizontal, in the lower part some- 

 what sagittal in direction. Hence when the ribs are elevated the upper 

 part of the thorax increases in its antero-posterior, the lower part in its 

 transverse diameters. At the same time, the lower portion of the sternum 

 is pushed forward and upward by the elevation of the anterior extremity 

 of the ribs and the widening -of the angle of the costo-chondral articulation. 

 With the elevation of the ribs there goes an eversion or outward rotation 

 which still further increases the transverse diameters. 



Coincidently with the increase in the transverse and antero-posterior 

 diameters of the thorax, and the partial removal of the pressure on the 

 lateral surfaces of the lungs there is also an additional expansion of the 

 intra-pulmonic air. As it expands it distends the lungs, causing them to 

 occupy the available space thus established. With the expansion of the 

 intra-pulmonic air there is a still further fall of pressure and an additional 

 inrush of air. Between the descent of the diaphragm and the elevation 

 of the ribs and the advance of the sternum the volume of air necessary 

 for the ordinary respiratory needs is introduced into the lungs. 



This elevation and outward rotation of the ribs is the resultant of the coop- 

 eration of the following muscles, viz.: the inter co stales externi, the intercar- 

 tilaginei, the levatores costarum, the scaleni and the serratus posticus superior. 



The action of the external intercostal muscles, as well as the action of 

 the intercartilaginei muscles, has been a subject of much discussion. Some 

 investigators have maintained that they are elevators of the ribs, and there- 

 fore inspiratory; others that they are depressors of the ribs, and therefore 

 expiratory in function. At the present time the general consensus of opinion 

 is that the former view is the one most in accordance with the facts. In 

 the following explanation as to their action, their relation to the ribs and to 

 the cartilages, must be recalled to mind. The relation of the external inter- 

 costals is such that the point of attachment of any given bundle of fibers 

 to the rib above lies nearer the vertebral column, nearer the fulcrum, 

 than the point of attachment to the rib below. The relation of the inter- 

 cartilagieni to the cartilages is such that the point of attachment of any 



