342 
Proceedings of the Royal Society 
momentum acquired by the pinion during its descent, and to 
prepare it for making the up stroke. It is elevated as a short lever 
to avoid the resistance of the air, and pushed away from the body or 
extended towards the end of the up stroke to prepare it for making 
the down stroke. It is depressed with great energy as a long 
lever , and hence the greater elevating and propelling power of 
the down as compared with the up stroke. 
When the bat and bird are stationary, the tip of the wing, from 
its alternately darting out and in, and forwards and backwards, 
during extension and flexion, and during the down and up 
strokes, describes an ellipse, the axis of which is inclined obliquely 
upwards and forwards. When the bat and bird are progressing at 
a high speed, the axis of the ellipse is inclined obliquely down- 
wards and forwards, the ellipse itself being converted into a spiral 
and then a wave line. The outward and forward (extension) and 
inward and backward (flexion) play of the pinion contributes to the 
balancing power of the bat and bird, as it augments the horizontal 
area of support. 
The wing of the insect is recovered or drawn towards the body, 
and that of the bat and bird recovered, flexed, and slightly elevated 
by the action of elastic ligaments. Those ligaments, by their con- 
traction, conserve and interrupt muscular efforts without destroying 
continuity of motion. 
The elastic ligaments are in many cases furnished with muscular 
fibres, and are most highly differentiated in those animals whose 
wings vibrate the quickest. 
The primary, secondary, and tertiary feathers of the wing of the 
bird are geared to each other by fibrous structures in such a 
manner that the feathers are made to rotate in one direction during 
flexion, and in another and opposite direction during extension. 
The double rotation of the feathers in question confers a distinctly 
valvular action on the wing of the bird. 
The under surface of the wing of the bat and bird is thrown into 
a beautiful arch during extension and the down stroke, the arch 
being so formed that its tension increases according to the pressure 
applied. 
The wing is inserted into the upper part of the thorax, and 
balances the body by playing alternately above, beneath, and on a 
