MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
531 
the above movements. The cartilages of the intermediate and diaphragmatic sets of 
ribs of the opposite sides are drawn much further apart, and the distance between 
them in front is much increased. The angle between the opposite conjoint cartilages 
of the sixth and seventh ribs below the sternum is enlarged. See diagram N 1.2. and 
fig. XII. b, inspiration, contrasted with fig. XII. a, expiration. 
On inspiration (see O 2, fig. XIV. b. ) the anterior extremities of the ribs ascend and 
move forwards, and carry forwards and upwards the cartilages and the sternum. The 
posterior angles and extremities of the ribs move backwards and downwards and push 
backwards, and lower the spinal column. As the longer intermediate ribs occupying 
the hollow of the arch move backwards, from their greater length, more than the 
other ribs, their immediate action is to deepen, and consequently shorten the dorsal arch. 
Diagrams. 
0 . 
1. Expiration. Lateral views. 2. - Inspiration. 
All those portions of the ribs in front of a line O 1.2, drawn from the insertion of the 
scalenus to the junction of the eleventh rib with its cartilage move forwards and 
ascend ; all behind that line move backwards. This observation can be readily tested ; 
place the thumb firmly on any part of the chest and breathe deeply ; if in front of 
the line indicated, the thumb will be carried forwards ; if behind the line, back- 
wards. 
On inspiration the costal cartilages (see diagrams P2, Q 2, compared with P 1, Q 1) 
curve forwards on themselves, and deepen the concavities behind them to each side 
of the sternum. Owing to the change in position of the ribs, their outer curves are 
throughout enlarged, and the posterior curves to each side of the spine are deepened 
(§§ 39. 40. 55). The spinal column is pushed more and more backwards from the 
first to about the sixth or seventh dorsal vertebra, and less to the first lumbar. The 
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