212 A MANUAL OF PHYSIOLOGY 



tense during inspiration, fix the first and second ribs (scalenus 

 anticus and medius, the first; scalenus posticus, the second 

 rib), and so afford a fixed line for the intercostal muscles to work 

 from on the lower ribs. 



The most important elevators of the ribs are the external 

 intercostals. The intercartilaginous portions of the internal 

 intercostals (the intercartilaginei muscles, as they are sometimes 

 called) also contract simultaneously with the diaphragm, and 

 may therefore be included in the list of inspiratory muscles ; but 

 instead of elevating the ribs they depress the costal cartilages, 

 and thus help to widen the angles between them and the ribs. 

 In addition to increasing the capacity of the chest, the con- 

 traction of the external intercostals and the intercartilaginous 

 muscles aids in inspiration by augmenting the rigidity of the 

 intercostal spaces, and so preventing them from being drawn in as 

 easily as would otherwise be the case when the thorax is expanded 

 by the action of the diaphragm 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) An 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 rota- 

 tion 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 



