RESPIRATION 



399 



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 estab- 

 lished. 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 intercostales 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 



B 



FIG IQO. DIAGRAMS ILLUSTRATING THE ACTION OF THE EXTERNAL INTERCOSTAL AND IN- 



TERCARTILAGINEI MUSCLES. 



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

 fore inspiratory; others that they are depressors of the nbs, and therefc 

 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. Ii 



rfSSdle of fiber, to the cutilage above lies nearer the 



study of the schematic model first presented by Hamberger 



