I70 REPTILES AND BIRDS. 



ing and elevating it in various degrees ; while a series of connections, 

 the fibres of which invest the base of the quills, curve round the 

 edge. 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. 



Besides flight, Birds possess other means of locomotion, being- 

 formed for walking, swimming, or flying, according as their habits 

 are aerial, terrestrial, or aquatic. Their general form, though pos- 

 sessing 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 closely covered with feathers, it is observed that the true 

 skin, or derma, is thin and transparent; when the reverse, 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, 

 which 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. 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, com- 

 pact, 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, called the barb, from which 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 dove-tailing, but by the anterior series of one barb over- 

 lapping and hooking into the re-curvate formation of the barb next 

 to it (Figs. 48, 49). The barbules themselves frequently throw out 

 filaments in the same manner, which are called barbicels, the object 

 of which is apparently the same — namely, that of connecting and 

 retaining the barbules in position. These barbules may be observed, 



