ESSENTIAL CHARACTERS OF BIRDS 75 



The bones which support these toes, the bones which answer 

 to those which contribute to form the sole of the foot in man, are 

 four in number. They are known as the metatarsals. The first 

 is detachable, and reduced to a mere vestige ; while the remaining 

 three have become welded together to form a single, cylindrical 

 shaft, known as the tarso-metatarsus, because the upper end of 

 this shaft is formed by the lower row of ankle bones or " tarsal " 

 bones. These, however, in the adult bird have become welded 

 one to another, and, with the long metatarsals, to form one solid 

 shaft. In the very young bird all these bones are separate, and 

 even in the nestling stage this compound nature can be traced. 

 For the row of ankle bones just referred to will be found forming 

 a separate plate capping the end of the shaft, while the shaft 

 itself will display traces of the three bones of which it is 

 made up. 



Some effort should be made to add the skeleton of the leg 

 of, say, a common fowl of about six to eight weeks old, to the 

 specimens used for teaching in the school museum. This com- 

 pound shaft, which forms the lower portion of the leg in the 

 living bird, is commonly clothed in scales and often brightly 

 coloured. 



Further, attention should be drawn to the fact that birds 

 walk upon their toes, and that what answers to the sole of the 

 foot in man forms what is commonly known as the " leg " in birds. 

 Only in a very few birds does the bird's heel ever come near the 

 ground, and no bird is really plantigrade, that is to say no bird 

 walks on the sole of its foot. 



The eyes of birds, it may be remarked, like those of reptiles, 

 are surrounded by a ring of bony plates, which in some species, 

 such as eagle and owls, are of considerable size. But these 

 plates do not, strictly speaking, form part of the true 

 skeleton. 



The Alimentary System. The organs which perform the work 

 of collecting, preparing, and digesting the food present but few 

 characters which can afford useful knowledge to those who are 

 seeking to acquire only the elements of ornithology. 



In studying this aspect of the birds' economy it must be borne 

 in mind that birds do not, and cannot, masticate their food 



