Miscellaneous. 815 
respiratory movements. But this would add unduly to the weight of 
the body. The lungs therefore are small, very porous, and light; 
yet nevertheless their efficiency is ensured by a more minute division 
of the capillaries, and a more complete exposure of these to the 
action of the air supplied so abundantly from the capacious air-sacs. 
In short, more perfect /ocalized instruments of respiration cannot be 
conceived. 
Our great physiologist, John Hunter, believed it impossible that 
the ribs and sternum of a bird could move while the powerful pec- 
toral muscles are engaged in flight. He therefore thought that the 
air-sacs of birds might be intended to act as reservoirs of air to be 
used in respiration during flight. These sacs, however, do not hold 
enough air to support the respiration of a bird for two minutes ; for 
in that time, if the trachea of a bird be tied, it dies; yet many birds 
continue on the wing for hours together. Sappey has endeavoured to 
explain the difficulty which occurred to Hunter by pointing out that the 
great pectoral muscles of birds arise exclusively from the sternum, and 
not at all from the ribs, as they do in mammals. But this explana- 
tion only removes 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 ster- 
num would be drawn powerfully upwards as the wings are drawn 
downwards, and so the sternum and ribs flexed. 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 in the direction of the axis of the coracoid ; 
so that they only draw the sternum and coracoid together more 
closely, and do not tend to flex these bones on one another. The 
common inspiratory muscles are therefore free to act, whether the 
pectorals are in action or not. ‘To be more exact, the line of action 
of the great pectoral muscle lies a little below the coracoid bone, and 
parallel to its axis. Hence, in contracting, the muscle will tend to 
depress the sternum, and so assist the inspiratory muscles, and ren- 
der inspiration deeper in flight than when the wings are closed. 
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, andthe 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 inter- 
costal 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. 
The hollow bones are filled with air, not for respiratory purposes, 
but to remove the moisture from the interior of the bones secreted 
by the endosteum, which would otherwise accumulate and defeat 
