RESPIRATION. 



389 



them downward against the abdominal walls, which advance and become 

 more convex. In forcible inspiration the diaphragm, acting from the central 

 tendon as the more fixed point, would draw the lower portion of the thorax 

 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 lung, 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 tower part some- 

 what sagittal in direction. Hence 

 when the ribs are elevated the upper 

 part of the thorax increases in its an- 

 tero-posterior, the lower part in its 

 transverse diameters. At the same 

 time, the lower portion of the sternum FIG. 190. DIAGRAM SHOWING THE POSITION 



k -miQViprl fnrwarH anH iir>warH Vv AND SHAPE OF THE DIAPHRAGM AT REST a AND 



. D y DURING INSPIRATION a' AND b.(Boruttau.} 

 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 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. 



