PLUMAGE. 185 



quills, curve round the edge of the tail. Their action is to spread 

 out the tail-feathers, and incline them to the right or left ; thus 

 enabling it to perform the part of a helm or rudder as it cleaves 

 the atmosphere. 



Besides flight, birds possess other means of locomotion. They 

 are formed for walking or for swimming as well as for flying, 

 according as their habits are aerial, terrestrial, or aquatic. Their 

 general form, though possessing all the characteristics of the class, 

 is modified and adapted to the kind of life they are intended to 

 lead. Where the skin of a bird is covered with feathers, it is 

 observed that the true skin, or derma, is thin and transparent ; 

 while the cuticle is thicker, and even covered with scales, in those 

 parts where feathers are absent. 



Before addressing ourselves to the physiological functions of 

 birds, a few words descriptive of their feathers, beaks, and claws 

 will not be out of place. 



The covering of birds is known by the general name of plumage. 

 It is composed of many individual feathers. The feathers are 

 horny productions, consisting of a hollow tube or barrel, and a 

 stem rising from it. Chemically, this covering is of the same 

 material as the hair on Mammals and the scales on reptiles and 

 fishes, differing only in its mechanical structure. Besides the more 

 conspicuous feathers, most birds have an underneath covering of 

 smaller ones known as down-feathers. A feather of the ordinary 

 kind consists of the tube, or barrel, by which it is attached to the 

 skin, varying in length according to the species ; the stem, or 

 shaft, composed internally of a soft, compact, but elastic substance 

 of a whitish colour, and in its buoyancy not unlike cork ; the web, 

 which is a lateral prolongation of the external coating of the shaft, 

 and which assumes the form of a thin linear membrane springing 

 from it at an angle more or less acute in different species : this is 

 the barb. From the upper edge of each barb two sets of minute 

 filaments proceed at an angle similar to that of the barb itself in 

 respect to the shaft. These smaller filaments are the barbules, by 

 means of which the barbs are retained in opposition not by the 

 barbules of one barb interlocking with those of another in the 

 manner of dovetailing, but by the anterior series of one barb over- 

 lapping and hooking into the recurvate formation of the barb next 



