508 
MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
Cl. C 2. 
While the obliquity of the vertebral ribs is from behind forwards and downwards, 
that of the sternal ribs is from before backwards and downwards. The motions of 
the sternal ribs on the sternum are precisely the same in principle with the motions 
of the vertebral ribs on the vertebrae, only they are all reversed. While the distant 
ends of the latter ribs when raised move forwards from the vertebrae, those of the 
former move backwards from the sternum ; while the upper edge of each vertebral rib 
glides backwards in relation to the lower edge of the rib above (§ 6, 7), that of the 
sternal rib glides forwards. 
The same holds as to the muscles; while the scalenus or levator costae (1) raises 
the vertebral rib, the sterno-costalis (3) elevates the first sternal rib ; while the ex- 
ternal intercostals (2) elevate and give the gliding motion to the vertebral ribs (§ 6, 7), 
the sternal intercostals (4) give the same motions to the sternal ribs ; and while the 
muscles of the vertebral ribs are directed forwards and downwards, those of the sternal 
ribs pass backwards and downwards. The two sets of muscles combine in one action 
to raise the ends by which the two sets of ribs articulate with each other (§ 6, 7)* 
19. On inspiration, the angles formed by the ribs are more open, the sternum and 
spinal column more distant. 
In diagram C 2, inspiration, the angle d.c formed by the articulation of the two ribs 
is much more open than in diagram C 1, expiration ; in the latter they are bent to a 
right angle, in the former they are nearly straight. As the costal ends of the ribs 
are more distant from the sternum and vertebrae in inspiration C 2 than in expiration 
C 1, so, to a double extent, is the distance increased between the sternum and the 
spinal column ; if the spinal column be fixed, the sternum moves forwards to a very 
great extent. 
