MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 227 



by augmenting the rigidity of the intercostal spaces, and so pre- 

 venting them from being drawn in as easily as would otherwise be 

 the case when the thorax is expanded by the action of the dia- 

 phragm and the other inspiratory muscles. 



Leaving out of account the floating ribs, which functionally form 



a part of the abdominal wall, the ribs in relation to their respiratory 



functions may be divided into the following groups: (i) The first 



^rib, which, moving itself very little, provides a fixed line towards 



which the next set of ribs may be raised. 



(2) Ar> upper costal series consisting of the ribs from the second 

 to the fifth. These are raised in inspiration towards the fixed first 

 rib by the contraction of the intercostal muscles. The movement 

 of these ribs is, mainly at any rate, a rotation around a transverse 

 axis, the axes on which they move corresponding to their necks. 

 The manner in which they are articulated to the vertebrae prevents 

 any sensible rotation around an antero-posterior axis or ' bucket- 

 handle ' movement. Since these ribs slant downwards and forwards 

 to their sternal attachments, the sternum is raised when they are 

 elevated; or, rather, since the manubrium is practically immovable 

 in ordinary breathing, the body of that bone is bent on the manu- 

 brium at the manubrio-sternal joint. 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 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 to increase the transverse diameter at any given level. 

 Such an increase is also favoured by the opening 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 external intercostals exerted on the 

 ribs, and the downward pull of the intercartilaginei and the resist- 

 ance of the sternum to further displacement exerted on the carti- 

 lages. The whole arrangement is perfectly adapted to permit the 

 expansion of the roughly conical upper lobes of the lungs. 



(3) The lower costal series, consisting of the ribs from the sixth 

 to the tenth. These ribs, with their muscles, form a mechanism 

 which normally acts along with the diaphragm (Keith). They are 

 so arranged that in inspiration the lateral and anterior part of 

 each moves outwards to a greater extent than the one above it. 

 There is not only a rotation around a transverse axis, by which the 

 lower end of the sternum, connected to these ribs by the combined 

 cartilages of the sixth to the ninth, is elevated, but also a rotation 

 around an antero-posterior axis. The movement of the lower ribs 

 results, therefore, in increasing both the back-to-front diameter and 

 the transverse diameter of the lower portion of the thorax. The 



