STRUCTURE OF BIRDS. 147 



majority of Birds, the first toe {hallux) (.'^4), 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 memhrana 

 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. — With 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 periods. 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 5;;astric juices are lodged. In this 

 antechamber the food is subjected 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. In 

 flesh-eating birds the stomach has thin walls and is much more 



capacious. 



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