7 
a part of the difficulty ; for the ribs are so articulated with the 
sternum, that they cannot move unless the sternum moves also. 
Now the sternum in respiration moves at its articulations with 
the two coracoid bones, these bones being fixed in regard to the 
sternum and humerus in the movements of flight. It might 
seem, therefore, that when the pectoral muscles contract, the 
sternum would be drawn powerfully upwards as the wings are 
drawn downwards, and so the sternum and ribs fixed. But this 
is not so; for the fibres of these muscles converge towards and 
pass over the coracoid bones on their way to be inserted into the 
ridge of the humerus, and they act, when the wing is extended 
in flight, in a line very nearly parallel to the axis of the coracoid, 
but a little below it. Their principal effect on the sternum, 
therefore, is to draw it more closely against the coracoid; so that 
they do not much interfere with the action of the respiratory 
muscles, rather assisting however the inspiratory muscles in 
depressing the sternum ; a circumstance favourable to deep inspi- 
ration during flight. 
The author gave a mathematical as well as an experimental 
proof that the external intercostal muscles raise both the ribs to 
which they are attached, and that the internal intercostals depress 
both ribs. A frame of wood, in the form of a parallelogram with 
hinges at the angles, represented two ribs, the spine, and the 
sternum. An india-rubber ring was passed over a peg in the 
upper rib and another in the lower rib, at different distances from 
the spine, to represent the intercostal muscle. Both ribs were 
elevated or depressed according as the upper peg was nearer to 
or further from the spine than the peg in the lower rib. : 
Dr Humpury made a few remarks upon some points in the 
paper, expressing himself not quite satisfied with the proof of the 
aeration of the blood by means of the double current of air from 
the cells. 
