MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
533 
On expiration, the sixth rib, or the upper of the intermediate set of ribs, is curved 
upwards, while the seventh is curved downwards. On inspiration, these two ribs move 
further apart, they each of them become parallel and assume an almost straight pro- 
file. The eighth or lowest of the intermediate set, and the ninth or uppermost of the 
diaphragmatic set of ribs, are on expiration curved downwards like the seventh, and 
like it they become more nearly straight on inspiration. The anterior ends of the 
sixth, seventh and eighth ribs, or the intermediate set, being linked together, are 
raised on inspiration exactly to the same extent, while the lateral portions of the 
seventh and eighth ribs are not so much elevated as is that of the sixth ; the sixth 
being raised more, relatively, in the centre than at the anterior extremity, while the 
seventh and eighth are more raised at their anterior extremities than at the centre. 
The curve of the profile of the ninth rib, or the first of the diaphragmatic set, is 
not so much lessened on inspiration as is that of the eighth, as the anterior extremity 
and centre of the ninth rib ascend to a proportional extent. The profile of the tenth 
rib is straight, while that of the two lower ribs is curved slightly upwards. The 
inner surfaces of the lower ribs, which in expiration are somewhat oblique, the ob- 
liquity looking upwards, take a vertical direction on inspiration. 
§§ 81 - 95 . Muscular actions. 
81. The great complication of the movements of the ribs in respiration is of course 
met by like complication in the actions of the muscles that move those ribs. 
82. The scaleni raise the first and second ribs and lower and bring forward the 
cervical vertebrae. See §72. 
The scaleni act to elevate the first and second ribs during the whole of an inspira- 
tion ; their action may be easily felt by placing a finger on each side of the neck, above 
the clavicles ; the fingers are gradually pushed outwards and further away from each 
other during the whole inspiration. While the scaleni act from the cervical vertebrae 
to raise the first rib, they, in turn, also act from the first rib to draw downwards and 
forwards the cervical vertebrae, § /2. This movement of the cervical vertebrae is not 
absolute but relative, for they, riding on the dorsal vertebrae, are carried backwards 
by the inspiratory movements of the latter; but the backward movement of the 
dorsal is considerably greater than that of the cervical vertebrae ; the latter indeed 
move forward relatively to the former, carry the origins of the scaleni more directly 
over the points of the ribs on which they act, and increase the dorsal arch. In the 
difficult breathing of persons suffering from chest disease, I have seen the head and 
neck move forwards on each inspiration. 
83 . The scaleni expand the cervical portion of lung. See § 71- 
In Man a considerable portion of lung is situated above the clavicles ; the expan- 
sion of this portion of the lung is effected, as it is in the Porpoise (see § 71), by the 
