544 MOVEMENTS OF INSPIRATION. [BOOK n. 



movements of the ribs may in normal breathing be almost 

 neglected. In the rabbit for instance, in normal breathing, almost 

 all the respiratory work is done by the contractions of the dia- 

 ' phragm. 



The descent of the diaphragm is effected by means of the 

 contraction of its muscular fibres. When at rest the diaphragm 

 presents a convex surface to the thorax; when contracted it 

 becomes much flatter, and in consequence the level of the chest- 

 floor is lowered, the vertical diameter of the chest being pro- 

 portionately enlarged. In descending, the diaphragm presses on 

 the abdominal viscera, and so causes a projection of the flaccid ab- 

 dominal walls. From its attachments to the sternum and the false 1 

 ribs, the diaphragm, while contracting, naturally tends to pull the 

 sternum and the upper false ribs downwards and inwards, and the 

 lower false ribs upwards and inwards, towards the lumbar spine. 

 In normal breathing, this tendency produces little effect, being 

 counteracted by the accompanying general costal elevation, and by 

 certain special muscles to be mentioned presently. In forced 

 inspiration, however, and especially where there is any obstruction 

 to the entrance of air into the lungs, the lower ribs may be 

 so much drawn in by the contraction of the diaphragm, that the 

 girth of the trunk at this point is obviously diminished. 



332. The elevation of the ribs is a much more complex 

 matter than the descent of the diaphragm. If we examine any one 

 rib, such as the fifth, we find that while it moves freely on its 

 vertebral articulation, it inclines when in the position of rest in an 

 oblique direction from the spine to the sternum ; hence it is 

 obvious that when the rib is raised, its sternal attachment must 

 not only be carried upward, but also thrown forward. The rib 

 may in fact be regarded as a radius, moving on the vertebral 

 articulation as a centre, and causing the sternal attachment to 

 describe an arc of a circle in the vertical plane of the body ; as the 

 rib is carried upwards from an oblique to a more horizontal position, 

 the sternal attachment must of necessity be carried farther away 

 in front of the spine. Since all the ribs have a downward slanting 

 direction, they must all tend, when raised towards the horizontal 

 position, to thrust the sternum forward, some more than others 

 according to their slope and length. The elasticity of the sternum 

 and costal cartilages, assisted by the articulation of the sternum 

 to the clavicle above, permits the front surface of the chest to be 

 thus thrust forwards as well as upwards, when the ribs are raised. 

 By this action, the antero-posterior diameter of the chest is 

 enlarged. 



Since the ribs form arches which increase in their sweep as 

 one proceeds from the first downwards as far at least as the 

 seventh, it is evident that when a lower rib such as the fifth is 

 elevated so as to occupy or to approach towards the position of the 

 one above it, the chest at that level will become wider from side to 



