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XXVI. On the Mechanism of Respiration. By Francis Sibson, Esq. 
Communicated by Thomas Bell, Esq., F.R.S., 8$c. 
Received December 18, 1845,- — Read March 12, 1846. 
l. MX or seven years since, while examining- the chest in persons subject to 
dyspnoea, I was struck by observing that the latissimus dorsi and the serratus 
magnus muscles acted during forcible expiration; on further inquiry, I found that 
neither of them acted during inspiration. I also ascertained that, on inspiration, the 
scaleni acted during the whole time ; that the superior ribs came nearer to each other, 
and the inferior moved farther apart ; and that the internal intercostal muscles 
between the six superior costal cartilages and the external intercostal muscles be- 
tween the superior ribs were in action. 
I communicated these observations to Professor Bell ; he advised me to examine 
and figure the muscles of respiration in the lower animals. This I did. 
After making an extensive series of dissections and illustrations of the respiratory 
apparatus in the animals that breathe with ribs, I exposed the muscles of respiration 
in the Dog and the Ass while alive, and observed what muscles acted on inspiration, 
and what on expiration. I afterwards inflated the lungs in the dead animal, and found 
that the same muscles that acted during inspiration were shortened when the lungs 
were distended. This was important, as it rendered the repetition of vivisections 
less necessary. 
Having extended these inquiries to all classes of animals breathing by ribs, com- 
mencing with the simplest, and ascending to Man, I came to the conclusion that the 
mechanical part of the function of respiration was far more complicated than it has 
generally been regarded, and that in some important particulars the generally received 
opinions on the subject required to be essentially changed. 
This series of researches form the subject of the following paper. 
REPTILES, §§ 2-16. 
2. The serpent tribe of reptiles, possessing no sternum, have ribs which are con- 
nected with the vertebrae only, being without costal cartilages. 
Snake, §§ 3-11, Plate XXIII. fig. I. a. b. 
3. In the Snake, the ribs, besides their respiratory action, serve as limbs for pro- 
gression. The question, What are its respiratory muscles ? becomes complicated with 
another question. What are its muscles of progression ? Do the same muscles serve 
both functions, or is each function performed by distinct muscles ? 
