532 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
posterior curves of the six or seven inferior ribs are deepened relatively to the spinal 
column, as is evidenced in diagram O 2, inspiration, compared with diagram O 1, 
expiration, in which the spines of the lower dorsal vertebrae are more concealed in 
the former than the latter by the posterior curves. The posterior curves of the 
diaphragmatic set of ribs are deepened more than those of the intermediate set, while 
those of the thoracic set, excepting the fifth rib, are not deepened at all ; the whole 
of the backward thrust of those ribs being spent in pushing back the spinal column. 
The twelfth rib does not move backwards so far as the eleventh, hence there is not 
here the usual gliding motion of the lower rib on the upper. Each rib has its own 
peculiar difference in curves and surfaces fitting it for its precise place, all the ribs 
combining, by change in position, to present one set of curves and surfaces in expi- 
ration, another in inspiration. 
The five superior ribs (the thoracic set) form as it were the dome and upper por- 
tion of the chest. On expiration, when the space they inclose is every way narrowed, 
the profile of the upper edge of each rib is concave (diagrams P 1 , O I , and fig. XIV. «.), 
the inner concavity is shallowed (diagram Q 1), and the oblique surface tends to the 
Diagram representing the position of a rib in expiration and in inspiration. 
P Expiration. Q 
P 1.2. Side views. Q 1.2. Views looking as it were down into Q 3.4. Front views. 
the chest, a transverse section. 
vertical direction (see diagram R 1). On inspiration, when the dome and upper part 
of the chest are amplified in every direction, the profile of the upper edge of each rib 
is nearly straight (diagrams P 2, O 2, and fig. XIV. b.), the inner concavity is deepened 
(diagram Q 2), and the direction of the oblique inner surface becomes more nearly 
horizontal (diagram R 2). 
