STRUCTURE OF BIRDS. ; 147 
majority of Birds, the first toe (hallux) (34), which is directed back- 
wards, being articulated with a short separate metatarsal (33). The 
number of joints of which the toes are composed increases from within 
outwards, from the first or hind toe possessing two to the outermost 
(37) which has five. 
This general description of the skeleton does not apply in every 
detail to all groups of Birds; some of them, especially the Ostrich-tribe, 
showing modifications of certain parts, the most important of which 
will be found exhibited in a special Case in the Gallery. 
Brain.—The brain is much more developed in Birds than it is in 
Reptiles, and entirely fills the spacious cranial cavity. 
Eye.—The power of vision is perhaps more developed in Birds than 
in any other vertebrate. The eyes are always of large size and pro- 
tected by two movable eyelids as well as a transparent membrana 
nictitans. The eyeball is strengthened by a broad ring of overlapping 
bony plates (Pl. XXV. fig. 6%.) which enables the bird to focus 
distant objects, and acts as a telescope. 
Ear.—The sense of hearing is very acute, but no external ear is 
developed, and the opening is hidden by the plumage. 
Smell, Taste, and Touch.—The senses of smell, taste, and touch are 
much more imperfect, but some Birds possess one or other of these senses 
in a higher degree than the rest; for instance, that of smell in the 
Apteryx, that of taste in the Parrots, and that of touch in the Snipes 
and Ducks. 
Digestive System.—W ith regard to the digestive system, it has already 
been noticed that teeth are invariably absent in existing birds, but were 
present in certain types of the Jurassic and Cretaceous pericds. The 
beak and the generally slender horny tongue are the organs of pre- 
hension, the former being frequently used to divide the food into small 
pieces. 
The gullet is long, like the neck, and generally dilated into a crop, 
where the food is stored, detained, and softened for a longer or shorter 
period. Before entering the stomach or gizzard, the gullet forms a 
second dilatation with thickened walls, known as the proventriculus, in 
which numerous glands secreting the gastric juices are lodged. In this 
antechamber the food is suhjected to the chemical action of the 
digestive process, whilst the stomach proper fulfils only a mechanical 
function. In birds feeding on vegetables, grain, etc., the walls of the 
stomach are extremely muscular, with a thick horny lining, which 
(assisted by small pebbles, purposely swallowed by the bird) forms a 
grinding apparatus capable of crushing the hardest seeds to pulp. Ju 
flesh-eating birds the stomach has thin walls and is much more. 
capacious. 
L2 
