On the mechanism of respiration / by Francis Sibson, Esq.

  • Sibson, F. (Francis), 1814-1876.
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: On the mechanism of respiration / by Francis Sibson, Esq. Source: Wellcome Collection.

    22/70 (page 18)
    §§ 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 intercostals 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. 47- 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.