MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
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into the chest, and F 3 and F 4 are front views of the rib of a Pig. E 2 shows the for- 
ward and backward thrust on inspiration, or increased distance between the sternum 
S and vertebrae V, and F 24 , show the lateral thrust. The sternum S and vertebrae V 
not admitting of further separation, the raised costal cartilage acts like a crank on 
the end of the rib jointed to it, and pushes it outwards. The area of the chest be- 
tween the opposite ribs is thus increased, and the elevation of the rib and cartilage 
is extended in two directions, one direction (E 2 ) to increase the depth, the other (F 2 ) 
the width of the chest. 
38. In the Pig, as we have seen, the elevation of the rib and cartilage is distributed 
in two directions, one direction forwards and backwards, the other outwards. The 
outward, or lateral thrust, is by far the greatest. There is in the Calf considerable 
forward movement of the sternum in addition to the lateral expansion. The lower 
costal cartilages are longer than the upper, consequently the lower part of the sternum 
is thrust further forward than the upper, exactly as it is in birds, and for the same 
reason (§ 22). 
In the Sheep, when the lungs are distended, the vertebrae are, I believe, pushed 
slightly backwards, but nearly the whole of the force is expended in the lateral 
thrust ; the chest, though but slightly deepened, is much widened. 
39. Inspiratory changes in the position of the curves and surfaces of the ribs. 
The space between the ribs is further increased by the change in profile of the 
curves of the ribs. In expiration the upper edge (diagram E 1 ) and the inner surface 
of the rib (diagram F 1 ) are each slightly curved. In inspiration (diagram E 1 ), when 
the rib is raised, the curve of the inner surface is very greatly increased at the ex- 
pense of that of the upper edge, the curve of which entirely disappears, giving place 
to a straight line; the form of the rib remains the same, but its profile and the direc- 
tion of its surfaces are changed. 
40. Lateral expansion of the chest from the varying profile of the curves of the ribs 
in the Ass. 
In the Ass, the unyielding cartilage is firmly fixed to the rib so as to form one 
piece with it. The whole of the lateral thrust is due to the different direction of the 
costal curves. Nearly the whole of the respiratory movements go to increase the 
width of the chest. The sternal end of each cartilage is a little raised, and the ver- 
tebral end of the rib is slightly depressed ; the sternum moves forwards and upwards 
with the cartilages ; the spinal column backwards and downwards with the ribs ; the 
increase in depth is very slight. 
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