ORGANS OF FLIGHT . 
261 
although the general principle on which the organs of flight are constructed is the same. In both 
(as indeed also in flying insects), this principle consists in having a strong framework, to which 
an up and down movement can he communicated, along the front of the wing, enabling it to strike 
the air with more or less force during its downward passage, whilst the effective surface of the organ 
is of a flexible or elastic nature, being formed in the bird by the long feathers which are implanted 
in the skin clothing the bones of the wing, and in the Bat by a thin leathery membrane which 
is stretched between the bones of the fore and hind limbs. Upon these leathern wings the Bats flit 
about noiselessly in the twilight or in the darkness of the night. They are able to advance with con- 
siderable speed, and also to turn and wheel about in their course with great facility. 
Of course, as in birds, the principal modification of structure exhibited in these animals is connected 
with their power of flight, and manifested in the fore-limbs. These, although most disproportionately de- 
veloped, still, however, display the same bones which have been described in the arms of the Monkeys 
and Lemurs, as will be seen in our figure of the skeleton of the European Mouse-coloured Bat. We 
find in them a strong humerus (a) of moderate length, articulating with large shoulder-blades (b), 
which cover a considerable portion of the back of the chest, and are kept apart by well-developed 
collar-bones (c), springing in front from a breast-bone (sternum, s), which, 
although distinctly showing Mammalian characters, projects in such a manner as 
to serve the purpose of the deep keel in the breast-bone of birds, and give attach- 
ment to the powerful muscles required to set the wings in motion (see accom- 
panying figure). The humerus is followed by the bones of the fore-arm (d), the 
radius and ulna, of which, however, the latter is generally very small, and reduced 
to a mere rudiment immovably fixed to the radius towards the end nearest the 
body. This section is the longest part of the arm, and the simplicity of its 
Structure is in connection with the fact that, as in birds, there is here no 
occasion for any movement of rotation in the arm, such as enables the tore-limbs 
of many Mammals to be applied to a variety of uses. At the extremity of the 
radius are the carpal or wrist-bones (e), which are small but numerous, and 
furnish surfaces for the articulation of the bones of the fingers. Ot these, the 
first, or thumb (/), is short, and composed of three joints, a metacarpal and two 
phalanges, the last of which bears a strong curved claw, of great use to the 
animal in clinging to various surfaces, and in walking on the ground. Of the 
other four fingers, the metacarpal bones (g) are very long and slender, forming, 
indeed, the greater part of the fingers ; they taper towards their tips, but at the 
tips themselves are slightly enlarged. The first, or index finger, in most Bats 
is composed of the metacarpal bone alone, but in some this is followed by two 
short phalanges. The other fingers possess either two or three phalanges. In general only the thumb 
possesses a claw, but in some Bats there is one also on the index finger. 
To convert this framework into an organ of flight its various parts are, as already stated, united 
by a membrane of more or less leathery appearance, although often so thin and delicate as to be 
somewhat translucent. It is an expansion or wide fold of the skin of the body like those foiming 
the parachutes of the flying Squirrels, &c. , and often called by the same name -jjatagvum. *W e 
shall employ the simple, if rather longer term, “ wing-membrane.” The bones of the arm, with then- 
accompanying muscles, and those of the fingers, are enclosed between the two layers of skin of v liicli 
the membrane is composed, and which they serve to extend and support. In front of the aim theie 
is a small portion of membrane filling up the angle of the elbow, and called the antebrachial membrane. 
The thumb is left free. Behind the arm is the great expanse of the wing, which springs from the 
sides of the body, and is also attached to the hind legs, generally extending down to the ankle. 
The wings are expanded by the spreading of the fingers, which radiate from the wrist something 
like the sticks of a fan. The second, or middle finger, which is the longest, runs to the extreme tip 
of the wing, but before reaching this it generally joins the extremity of the first, or index finger, which 
thus acts as a sort of stay to it, and the two fingers together form a tolerably stiff support for the 
outer margin of the wing. The other two fingers (the third and fourth) traverse the wing to its 
hinder border, where they carry out the membrane into small pointed projections ; so that when the 
34 
THE STEllNCM OF FLYING 
FOX (PTEROPUS). 
