1872.] Natural and Artificial Flight. 189 
delivered downwards and forwards, and not as previous investi- 
gators have stated, vertically, or even slightly backwards.* 
. - . To confer on the wing the multiplicity of move- 
ment which it requires, it is supplied at its root with a double 
hinge or compound joint, which enables it to move not only 
in an upward, downward, forward, and backward dire¢tion, but 
also at various intermediate degrees of obliquity. . . . 
The wing of the bat (Fig. 2) and bird (Figs. 3 and 4), like 
that of the insect, is concavo-convex, and more or less 
twisted upon itself. The twisting is in a great measure 
owing to the manner in which the bones of the wing are 
twisted upon themselves, and the spiral nature of their 
articular surfaces, the long axes of the joints always inter- 
secting each other at nearly right angles. As a result of 
this disposition of the articular surfaces, the wing may be 
e 
Fic. 3. Fic. 4. 
Fig. 2. Right wing of the Bat as seen from behind and from beneath. 
When so regarded, the anterior or thick margin (d f) of the wing 
displays different curves from those met with on the posterior 
(0 c) or thin margin, the anterior and posterior margins crossing 
each other, as in the blades of a screw propeller. 
Fig. 3. Left wing of Heron, partially extended, seen from beneath and 
from behind,—shows the spiral configuration and crossing of the 
anterior (d e f) and posterior (c a b) margins of the pinion; e, 
anterior axillary curve pointing downwards; f, posterior axillary 
curve pointing upwards; c a, posterior axillary curve pointing 
upwards; 8, posterior distal curve pointing downwards. The 
posterior axillary and distal curve are reversed in complete 
extension (compare 0 ac of the present Fig. with b ac of Fig. 4. 
shot out or extended, and retra¢ted or flexed in nearly the 
same plane, the bones of the wing rotating in the direCtion 
of their length during either movement. This secondary 
action, or the revolving of the component bones upon their 
own axes, is of the greatest importance in the movements 
of the wing, as it communicates to the hand and forearm, 
and consequently to the volant membrane, or to the primary 
* The up stroke when the body is progressing at a high horizontal speed 
is delivered upwards and forwards, so that the wing invariably aé@s obliquely 
after the manner of a kite. Whether the wing is made to vibrate vertically 
or horizontally, it, practically speaking, in progressive flight, strikes downwards 
and ogra during the down stroke, and upwards and forwards during the 
up stroke. 
