170 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 aérial, 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 A/umage, 
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 ¢wée or barrel, 
by which it is attached to the skin, varying in length according 
to the species ; the séem or shaft, composed internally of a soft, com- 
pact, but elastic substance of a whitish colour, and in its buoyancy 
not unlike cork; the zed, 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 darbudles, 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 Jarbicels, the object 
of which is apparently the same—namely, that of connecting and 
retaining the barbules in position. These barbules may be observed, 
