( 2 ) 
Their anterior extremities, destined to support them in flight, 
cannot serve either for resting on or grasping; they are con- 
sequently hiped, and take objects from the ground with their 
mouth : the body being necessarily inchned forwards beyond their 
feet, the thighs are in advance, and the toes are elongated to form 
a sufficient basis. The pelvis or basin is much extended in length, 
to furnish an adequate surface for the attachment of the muscles 
which support the body on the thighs. There is also a suite of 
muscles going from the basin to the toes, and passing over the 
knee and heel, so that the mere weight of the bird curves or con- 
tracts the toes; it is thus that they are enabled to sleep perched 
upon one foot, (i) The ischions, and especially the pubes, are 
prolonged backwards^ and are placed wider apart, to leave the 
space necessary for the developement of the eggs. See PI. i. 
The neck and beak are elongated to enable ihem to reach the 
earth, but the former has sufficient mobility to bend backwards 
when the bird is standing still; it has, therefore, many vertebrae: 
on the other hand, the trunk, which merely serves as a support 
to the wings, is necessarily moveable only in a small degree. The 
sternum (to which the muscles which lower or depress the wing 
to strike the air in flight are attached) is of a very great extent, 
and its surface is further augmented by a plate or kcely projecting 
charged with more oxygen, which we know to be the powerful sup- 
porter of respiration, since a small animal, when confined in oxygen 
gaS; lives thrice as long as another confined in the same hulk of common 
air. From imbibing a much greater proportion of this vivifying prin- 
ciple, Birds enjoy a much greater irritability ; that is, their muscular 
force and digestive powers are greater, and all their efforts and 
actions are more energetic, instantaneous, and vigorous than those of 
larger animals whose construction does not afford these cavities. Irri- 
tability depends on respiration and circulation ; sensibility on the brain 
and nerves. It is believed that the high temperature which the bodies 
of Birds attain, especially Avhen sitting, and that extreme force which 
they employ in the act of flying, continued for several days, both de- 
pend on the action of the air on the blood. 
(1) The course of these muscles from the ilium to the nails, is traced 
in the leg of a common Sparrow, fig. 14. It is evident, that when the 
angles a a are lessened (on the femur, tibia, and tarsus, /"it 7' becoming 
more nearly parallel to each other by the stooping of the bird on the 
perch), the tendons are strained to pass over the additional length 
created by tlie greater intervals, b c, between the joints, and the toes 
to which these tendons are fixed are cramped by the action. There is 
generally a distinct tendon passing to each point of the toes, and they 
unite above the knee. From the ilium to this point of union it is a 
muscular apparatus, but from thence it becomes tendon. 
osr 
