MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
517 
Diagram I 1 . Expiration. Diagram I 2 . Inspiration. 
Section of the three superior ribs and their intercostal muscles (from the Ass) . 
I 1 . Expiration. I 2 . Inspiration. 
1 r. First rib. 2 r. Second rib. 
e. External intercostal, i. Internal intercostal. 
42 . Diaphragmatic set of ribs. 
The posterior angles of the free or diaphragmatic set of ribs, whose cartilages are 
floating, are drawn considerably backwards. They thus increase the area between 
the opposite ribs in the manner demonstrated in the Snake (diagrams B 1.2. § 9 ). 
The space in front between the cartilages of the two sides is widened. The upper 
edge of each of these ribs, except the two or three lowest, glides backwards in relation 
to the lower edge of the rib above 6. 7)- The gliding motion that would result from 
the elevation of the two or three lower ribs (diagram A 1.2) is, I believe, quite 
neutralized by the greater degree to which the rib immediately above each of them 
is drawn backwards. Corresponding with the gliding movement of rib on rib, each 
cartilage glides downwards and backwards on that above it; each cartilage forms 
also a less acute angle with the rib. 
43 . Intermediate set of ribs. 
The intermediate or neutral set of ribs (two in the Sheep and Pig, three in the Ass), 
whose cartilages united to each other articulate with the lower end of the sternum, 
neither approach to nor recede from each other during inspiration. The elevation 
of the superior rib acts to raise the rest. The bodies of the ribs move outwards, 
their posterior portion backwards, their cartilages slightly forwards, and the angle 
between the cartilages of the two sides below the sternum is enlarged. These inter- 
mediate ribs partake of the motions both of the ribs of thoracic respiration superior 
to them, and of the diaphragmatic ribs inferior to them. The distance between the 
lowest of the intermediate set of ribs and the highest of the diaphragmatic set is 
greatly increased on inspiration. 
In the Ass, the lowest costal cartilages articulating with the sternum are so firm, 
that they form, with the set of three intermediate ribs, one unyielding piece. 
