MR. SIBSON ON THE MECHANISM OF RESPIRATION. 
521 
below the centre of motion of the ribs ; the fibres fixed into those ribs act in relation 
to the ribs from below upwards and forwards to draw them downwards, and slightly 
backwards. We have "here this curious fact, that the superior fibres of the same 
muscle, the serratus magnus, are expiratory, while the inferior fibres are inspiratory. 
The action of this muscle can only be demonstrated on the living body, but we might 
have inferred, a priori, on knowing the relative direction of the various fasciculi of 
the serratus and of the ribs into which they are respectively inserted, which fibres are 
expiratory and which inspiratory. 
The serratus is in full stretch when the animal stands, and then it acts from the 
ribs on the base of the scapula : if all its fibres were expiratory, it is clear that when 
the scapular action is called into play, as the expiratory action would be constant, 
inspiration would be interfered with. If all were inspiratory, the expiratory action 
would be clogged. As it is, the central fibres which support the scapula are neutral, 
the superior expiratory, the inferior inspiratory, and by this beautiful adjustment every 
action of the muscle is unembarrassed. 
52 . Serratus posticus superior. 
The whole of the eight fibres of the superior serratus posticus I found to act during 
inspiration. 
53 . Serratus posticus inferior, superior fibres expiratory , inferior inspiratory . 
The four lower fibres of the serratus posticus inferior acted during inspiration to 
draw backwards the four inferior ribs, the two superior fibres of the same muscle 
acted, during expiration, on the eleventh and twelfth ribs to draw them downwards, 
while the fibres acting on the thirteenth and fourteenth ribs appeared to be neutral. 
The tendency of all the fasciculi of the inferior serratus would be from their direction 
to draw the ribs both downwards as in expiration, and backwards as in inspiration ; 
the proportion in which the four lower ribs are drawn backwards overbalances that in 
which they are drawn downwards, and their fibres are inspiratory ; but the proportion 
in which the eleventh and twelfth ribs are drawn downwards overbalances that in 
which they are drawn backwards, and their fibres are expiratory ; as the downward 
and backward motions of the thirteenth and fourteenth ribs balance each other, the 
fibres acting on those ribs are neutral. 
54 . Expiratory muscles. 
I observed the following muscles to act during expiration : the rectus abdominis, 
which draws down the sternum slightly, and the fibres inserted into the first rib ari- 
sing from the tendon of the rectus ; the external oblique, which pulls downwards 
and forwards the diaphragmatic ribs and a portion of the superior ribs ; the inferior 
oblique, which draws down the lower ribs and their costal cartilages ; the transver- 
sales, which draw nearer to each other the diaphragmatic ribs of the opposite sides, 
contracting the abdomen ; and the sacro-lumbalis. 
3 Y 
MDCCCXLVI. 
