MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
545 
At first sight the opposite action of the different fibres of the same muscle seems 
contradictory and unnatural, but when we find that the ribs at the upper part of the 
chest approach, whilst those of the lower part recede from each other, and that the 
posterior portion of each rib moves backwards, while the anterior portion moves 
forwards, we admire the perfect subservience of the actions of the muscles to the mo- 
tions of the ribs. We now wonder that the motion of the different ribs, and of the 
same rib in different parts, should be so opposite; but when we find that the mo- 
tions of these ribs and of the diaphragm exactly correspond to the form and capacity 
of the lungs in different animals, and that the form and capacity of the lungs are in 
each exactly fitted to the habits of the animal, we feel that we have some insight into 
the most beautifully adjusted and harmonious piece of machinery that either art or 
nature has constructed. 
Description of the Plates. 
The same figures denote the same parts in all the illustrations, both in the engraved 
figures and in the drawings deposited with the paper in the Archives of the Royal 
Society. 
1 . The Scalenus of the first rib, present in all, voluminous in the Porpoise (fig. VII.), 
trivial in the Seal (fig. VIII. §§ 10.23.47.58.71.72.82.83). 
1 a. A scalenus of the first rib, peculiar to the Porpoise (fig. VII.), external to the 
large funnel-shaped scalenus (§§ 71-72). 
1 a. In Man (fig. XIII. b.), a funnel-shaped scalenus of the first rib often exists, 
acting to expand the apex of the lung (fig. XXVI. Archives of the Royal Society) 
(§84). 
2. A posterior scalenus of the first rib, present in the Otter, Calf and Rabbit (figs. 
XII. IX. XI.), and the Monkey (fig. XV. in the Archives of the Royal Society). 
2 a. An additional insertion of No. 1. passes in the Calf (fig. IX.) to the second rib. 
3. A posterior and external scalenus of the first rib, inserted near the head of the 
bone, external to the levator costse, exists in the Calf and Dog (figs. IX. X.), and the 
Monkey and Ass (figs. XV. X. Archives of the Royal Society). 
4. The long anterior scalenus to several ribs, exists in the Dog, Otter, Seal, Rabbit 
and Calf (figs. X. XII. VIII. XI. IX.), and in the Monkey (fig. XV. Archives of the 
Royal Society) (§§ 47-58). 
5. The first levator costae. 
6. The levatores costarum (§§ 7-9.23. 50. 86). 
7. The external intercostals (§§ 6.7.24.46.48.49.60.85). 
7«. In Birds (figs. III. VI.), portions of the external intercostals arise from the 
spurs (§ 23). 
4 B 
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