6 
BIRD-LIFE. 
grey. Under the microscope we can detect, even in one 
and the same feather, several distinct modes of construc¬ 
tion, each agreeing with a certain shade of colour. From 
this it follows that the beauty of a feather depends less 
upon colouring matter than upon the disposition of the 
rays of light in reference to the construction of the 
feather itself. 
No matter how singular the structure of a feather 
may he, it is always absolutely necessary to the existence 
of the bird. It gives flight, maintains the proper tem¬ 
perature of the blood, lends softness to the form and 
beauty to the colouring. The feather makes the bird a 
bird, as much as the hand makes a man a man. The 
possession of feathers and the power of inflation are the 
two most essential endowments of the bird’s body, and 
they belong to it alone. 
But there are still other characteristics distinguishing 
the body of a bird from those of other animals, which we 
have now to consider. The framework of the thorax is 
remarkable for extreme firmness and immobility. These 
properties are both due to a remarkable elongation of the 
sternum or breast-bone, and the close connection of the 
dorsal vertebrae with the sternum as well as with one 
another. This bone shields the whole chest, like a breast¬ 
plate, and covers a part of the abdomen as well. It is 
attached without any cartilage to the sternal ribs. Along 
the centre of the thorax runs a strong keel, absent in the 
Struthionidce , to which are attached the pectoral muscles. 
The ribs are provided with peculiar processes, which link 
them together. The vertebral column is as flexible at its 
extremities as it is rigid in the centre; the vertebrae them¬ 
selves vary considerably in number. In mammals, gene¬ 
rally speaking, the length of the neck is determined by the 
