518 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
§§ 44-54. Muscles acting on the Ribs. 
44. The actions of the costal muscles are entirely subservient to the motions of 
the ribs. We cannot comprehend the actions of the muscles unless we rightly under- 
stand the movements of the ribs. 
45. In Birds and in Snakes all the external inter costals are inspiratory , and all the 
internal intercostals are expiratory . 
In birds, still more so in the Snake, the ribs are similar throughout and they all 
go through analogous motions. The snake has no sternum and sternal ribs; the 
bird has no set of diaphragmatic ribs with floating cartilages ; all its lower vertebral 
ribs articulate with sternal ribs. All the external intercostals are inspiratory, all the 
internal expiratory (§§ 6-8). 
46. In Mammalia the superior intercostals have the opposite action to the inferior, be- 
cause the superior and inferior ribs have opposite motions. 
When I exposed the muscles of the living and breathing Ass, I was surprised to 
find that while the superior external intercostal muscles were invariably inspiratory, 
the inferior were as invariably expiratory, and that while the posterior fibres of the 
same muscle were inspiratory, the anterior were expiratory. These were the facts ; 
I could not gainsay them. It was only after much inquiry that they became intelli- 
gible, not until I had comprehended the varying movements of the ribs at different 
parts ; then everything, step by step, became lucid. 
The sole duty of the muscles is to move the ribs. It is not because two muscles, 
between two distinct pairs of ribs, have the same direction of fibres and the same 
name, that they perform the same functions ; it is because the two sets of ribs 
go through the same motions simultaneously. If one set of ribs, the superior, 
approach each other, while the other, the inferior, go further apart, on each inspira- 
tion, ought we not, a priori, to infer that the two sets of muscles passing between the 
two sets of ribs will have different functions? that if the muscle between two superior 
ribs that approach each other be inspiratory, that between two inferior ribs that recede 
from one another will be expiratory? In truth we ought to infer this, for the fact is 
so. In fact it is not the system of muscles, but the system of ribs that we must regard. 
4 7- Scalenus. 
In the Pig, the Sheep and the Calf, in addition to the scalenus of the first rib, there 
is another scalenus that acts on a few of the superior ribs to raise them on their outer 
surface. The Ass possesses no scalenus save that of the first rib. This scalenus 
scarcely raises the first rib at either end, but it raises the rib at the centre, and con- 
verts the upward into an inward curve. 
