RESPIRATION 201 



end of that bone is practically immovable in ordinary breath- 

 ing, its lower extremity is tilted forwards. This causes an 

 increase in the 'antero-posterior diameter of the thorax. 

 Further, since the arches formed by the ribs widen in regular 

 progression from above downwards, at least in the upper 

 portion of the thoracic cage, so that the second rib is a 

 segment of a larger circle than the first, and the third than 

 the second, it is clear that a general elevation of the chest 

 will tend also to increase the transverse diameter at any 

 given level. Such an increase is also favoured by the open- 

 ing out of the angles between the bony ribs and the costal 

 cartilages under the influence of the couple (or pair of 

 oppositely directed forces) that acts on them viz., the 

 upward pull of the levatores costarum and the other elevators 

 exerted on the ribs, and the resistance of the sternum to 

 further displacement exerted on the cartilages. The widening 

 of the thorax from side to side may also be in a slight degree 

 ascribed to a twisting movement of the ribs, which tends to 

 evert their low r er borders. 



Expiration in perfectly tranquil breathing is brought about 

 with very little aid from active muscular contraction. The 

 sense of effort disappears as soon as the chest ceases to 

 expand. The diaphragm and the elevators of the ribs relax. 

 The structures that have been stretched or twisted recoil 

 into their original positions ; the structures that have been 

 raised against the force of gravity fall back by their weight, 

 and in the measure in which the pressure increases in the 

 thoracic cavity the elasticity of the lungs causes them to 

 shrink. The pressure in the alveoli, which at the end of 

 inspiration was just equal to that of the atmosphere, is thus 

 increased, and the air expelled, It is possible that, even 

 in man and in quiet respiration, a slight contraction of the 

 abdominal muscles hastens the return of the diaphragm jto its 

 position of rest, and that the triangularis sterni helps in 

 depressing the costal cartilages. In reptiles and birds, 

 expiration is normally effected by an active muscular con- 

 traction. This is also true in some mammals the rabbit, 

 for instance, in which the external oblique muscle of the 

 abdominal wall takes an important share in the expiratory act. 





