AVES. 
153 
trees. A set of muscles take their rise in the pelvis and extend 
themselves to the toes, so that the weight of the bird alone is 
sufficient to close them. 
The lower extremities are composed of a femur, a tibia, and 
a peroneum which is articulated to the femur by a spring, whose 
extension is maintained without the muscles being exerted. The 
tarsus and metatarsus consist of a single bone only. 
Most birds undergo two moults annually; and in some species 
the winter plumage differs considerably from that of the summer; 
and the male and female also vary in colour, in many species. 
The digestive function in birds is very rapid. The stomach 
is composed of three parts; namely, the crop, the succentorial 
ventricle, and the gizzard. 
CLASSIFICATION OF BIRDS. 
The orders and genera into which birds are divided, are esta¬ 
blished chiefly from the formation of the bill and feet. The 
classification adopted is that of Temminck, who possesses a 
deeper knowledge of this subject than any other naturalist. 
It is remarkable for its simplicity. He divides this class into 
sixteen orders, namely,— 
I. Rapaces. —Birds of prey. 
II. Omnivores —Those which live on all kinds of food. 
III. Insectivores. —Birds which feed on insects. 
IV. Granivores. —Birds which feed on grain. 
V. Zygodactyli. —Birds which have two toes before and 
one behind. 
VI. Anisodactyli —Birds which have the exterior toe joined 
to the middle one at the base. 
VII. Alcyones. —Birds with three toes before, united, and 
one behind; the tarsi being very short. 
VIII. Chelidones. —Birds with short legs, three toes be¬ 
fore, divided, or only united at the base by a short membrane; 
the back toe often reversible. 
IX. ColumbjE.—B irds with three toes before, entirely divid¬ 
ed, and one behind. 
X. Galling. —Birds with three toes before, united by a 
