AVES. 283 
that there are two toes in front and two behind. In the Swifts, 
again, all the four toes are turned forwards. In many of the 
swimming birds (/Va¢azores) the hinder toe is wanting or rudi- 
mentary ; and in the Ostrich both this and the next toe are 
absent, so that the foot consists of no more than two toes. 
The digestive system in Birds consists of the beak, tongue, 
gullet, stornach, intestine, and cloaca, with certain accessory 
glands. There are no teeth, and the beak is employed, in 
different birds, for holding and tearing the prey, for prehension, 
for climbing, and in some cases as an organ of touch, being 
in these last instances more or less soft, and supplied with 
nervous filaments. In many birds, too, the base of the bill 
is surrounded by a circle of naked skin, constituting what 
is called the “cere,” and this too serves as an organ of 
touch. The tongue of birds can rarely be looked upon as an 
organ of taste, since it is generally cased in horn, like the man- 
dibles. It is principally employed as an organ of prehension, 
but it is soft and fleshy in the Parrots, and in them, doubtless, 
acts as an organ of taste. Salivary glands are always present, 
but they are rarely of large size, and are often of extremely 
simple structure. In accordance with the length of the neck, 
the gullet is usually very long in birds, and is generally very 
dilatable. In the flesh-eating and grain-eating birds the gullet 
is dilated (fig. 153, c) into a pouch which is called the “crop,” 
and is situated in the lower part of the neck, just in front of 
the merry-thought. This may be simply a dilatation of the 
tube of the gullet, or it may be a single or double pouch. The 
function of the crop is to detain the food, for a longer or shorter 
period according to its nature, before it is submitted to the 
action of the proper digestive organs. In the Pigeons, the food 
which has been previously softened in the crop is returned to 
the mouth, and supplied to the young in a state suitable for 
digestion. The gullet, after leaving the crop, shortly opens 
into a second cavity, called the “proventriculus,” which is the 
true digesting stomach, and is richly supplied with glands 
which secrete the digestive fluid or gastric juice (f). This, in 
turn, opens into a muscular cavity which is called the “ giz- 
zard” (g), and which leads into the commencement of the small 
intestine. The characters of the gizzard vary with the nature 
of the food. In the birds of prey, which live on an easily- 
digested animal diet, the walls of the gizzard are thin and 
