ESSENTIAL CHARACTERS OF BIRDS 73 



The Skeleton. As touching the skeleton of birds only a few 

 words are necessary. In the first place, it should be remarked 

 that birds' bones are exceedingly light, being more or less filled 

 with air supplied from the air-sacs ; but the bones of the wing 

 and legs, in some birds, such as the goose tribe, and the swallow, 

 for example, are filled with marrow. If the bones of any part 

 of the skeleton which lodges air be examined, large cavities will 

 be found therein these are the air chambers. 



Birds are remarkable for the great length and flexibility of the 

 neck, which enables the head to reach every part of the body 

 whereby the feathers may be " preened " and kept in good order. 

 The fact that the skull joins the neck by means of a single ball and 

 socket joint only instead of by a double joint as in mammals 

 (Fig. 3, p. 20) (dog, cat) adds still further to its mobility. 

 The number of separate bones in the neck varies from twelve 

 to twenty-four. In mammals there are but seven vertebrae, 

 alike in the short - necked whale and the long - necked 

 giraffe. 



The fore-limb has undergone great modifications, in so far as the 

 hand is concerned (Model C. 8-9). Only three fingers are present. 

 Of these the first or thumb (Model C.) is reduced and a mere 

 stump, while the second and third are welded together to form 

 a double rod (Model C. 9), but finger number 3 has become greatly 

 reduced. A reference to text (Fig. n) will make this more 

 apparent. This transformation of what was once a five-fingered 

 hand has come about the better to support the great " quill " 

 or " flight-feathers " (Fig. 23), this limb being no longer used 

 for the support of the body when on the ground. 



The breast-bone and the bones of the shoulder-girdle have 

 undergone great changes to enable them to support the wings during 

 flight. 



The breast-bone, which is slung on to the body by means of 

 the ribs, is a broad plate of bone, down the middle of the under 

 surface of which there runs a long " keel " to which the great 

 breast-muscles are attached, which move the wings. 



The shoulder-girdle bones are concerned with the support of 

 the wings. They comprise the scapula or blade-bone (Model C. 6), 



