(5 ) 
temperature to which their movements expose them. The aerian 
cavities which occupy the interior of their hody, and even the place 
of the marrow in the hones, diminish their specific gravity, (i) 
The sternal portion of the ribs is ossified like the vertebral, in order 
to give more force to the dilatation of the breast. (2) 
The eye of Birds is disposed so as to distinguish near and distant 
objects equally well. A vascular folded membrane, which pro- 
ceeds from the bottom of the globe to the edge of the chrystalline, 
probably contributes to this property, by displacing the lentil. 
The anterior face of the globe is strengthened by a circle com- 
posed of bony pieces ; and besides the two ordinary eye-lids, there 
is always a third placed in the inner axngle, which, by means of a 
curious muscular apparel^ can cover the front of the eye like a 
curtain. (5) The cornea is very convex, but the chrystalline is 
flat, and the vitrea small. 
The ear of Birds has but one bone between the tympanum and 
the fenestra ovalis ; (4) their helix is a cone, scarcely arched; but 
their semi-circular canals are large, and lodged in a part of the 
skull, where they are environed on all sides by aerian cavities, 
which communicate with the OS quadratum. The nocturnal birds 
alone have a large exterior conch, which, however, does not 
project like that of quadrupeds; it is generally covered with feathers 
with finer beards than the others. (5) 
The organ of smell, hid in the base of the beak, has generally 
(1) The purpose of this organisation, however, is not well under- 
stood ; it is not to aid their flight, as we might at first suppose, since 
the Ostrich and the Casoar are full of these aerian cavities, and yet 
scarcely fly at all; whilst the Genera Larus and Anus, (with the ex- 
ception of the common Duck) and Birds which hunt on the surface 
of waters, whose frames are comparatively deficient in these cavities, 
(6§r4^'r rj and whose bones are full of marrow instead of air, fly ex- 
ceedingly well. 
(2) It will be recollected that this part of the ribs is only cartila- 
ginous in Mammalia. 
(3) This, which is called the nictitating membrane, being trans- 
lucent, defends the eye of the bird when flying in face of the sun, 
without obstructing the sight. It is for the same purpose, in fact, as 
the coloured glass in our instruments for observing the sun. It also 
protects the eye of the Diving Birds, when under water. 
(4) In Mammalia there are three, and they are articulated angu- 
larly. 
(5) We must lay back the feathers with our fingers to discover the 
conch, which is a thin leathery piece of flesh, in sliape not unlike our 
own ; but the orifice of the ear is behind instead of before it. 
