542 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
If any part contiguous to the ribs, on one side, would be injured by the respiratory 
motion of those ribs, then that side of the chest is often motionless, though the lung 
be sound. 
If the air-cells be dilated, the whole chest takes on permanently the form that it 
has on a deep inspiration. 
These instances show the practical value of a thorough knowledge of the healthy 
respiratory movements of each portion of the chest ; the attention is at once called 
to any point that, owing to disease, has not its due motion. 
The above paper relates almost exclusively to the mode in which the movements 
of the ribs expand and contract the chest in respiration ; the diaphragm is only 
treated of incidentally. The remarkable series of respiratory movements that take 
place in the nostrils, lips, mouth, palate and tongue, in the larynx and pharynx, and 
in the cervical fasciae, are not even mentioned. 
On a future occasion I hope to investigate those parts of the mechanism of respi- 
ration that are here only briefly alluded to, or altogether omitted. 
SUMMARY. 
Demonstrative proof of the statements in the following summary is, I believe, ad- 
vanced in the preceding paper and its accompanying drawings. 
The Snake (§§ 3-11. Plate XXIII. figs. I. II.) has the most elementary form of 
ribs. They are all alike, and are only attached at their vertebral end, their anterior 
ends being free (§ 4). There are neither sternum nor costal cartilages. 
When the lungs are expanded, the levatores costarum, and the external intercostals 
raise the ribs, widen the spaces between them, make their anterior ends move for- 
wards, and cause the upper edge of one rib to glide backwards in relation to the 
lower edge of the rib above it. Diagram A 1.2, fig. I. a. b. §§ 4. 6. 7 - 10. The scalenus 
acts on the first rib. 
The levatores costarum draw the ribs backwards behind, outwards to the side. 
Diagram B 2, § 9. 
When the lungs are emptied the movements of the ribs are exactly reversed by 
the long depressors of the ribs, and by the internal intercostals and the transversalis. 
Diagram A 1 , B’, §§ 6, 1 1. 
In Birds (§§ 1 7-27* Plate XXIV. XXV. figs. III.— VI.) a sternum and sternal ribs are 
coupled with the vertebral ribs and vertebrae to complete the circuit of the chest (§ 17 ). 
The respiratory movements, and the muscles exciting the movements of the sternal 
ribs on the sternum and on each other, are exactly the same in principle, though 
reversed in direction with those of the vertebral ribs on the vertebrae and on each 
