AVES. 277 
CLAP aART SOE 
CLASS IV. AVES. 
THE fourth class of the Vertebrates is that of the Birds or Aves, 
which may be shortly defined as being “ oviparous Vertebrates, 
with warm blood, a double circulation, and a covering of fea- 
thers” (Owen). The other leading characters which separate 
the Birds from the other Vertedrata are that the red blood-cor- 
puscles are nucleated, the skull articulates with the spine by a 
single articulating surface (or condyle), the breathing-organs are 
in the form of lungs, which communicate with a variable num- 
ber of air-sacs scattered through the body, and the fore-limbs 
are never terminated (in existing birds) by more than two fingers, 
ending in claws, and are generally modified so as to form “wings” 
or organs of flight. 
The feathers, which form such a distinctive character of birds, 
are formed by a modification of the outer layer of the skin (epi- 
dermis), and from their non-conducting nature they serve to 
maintain the high temperature of the body which is so charac- 
teristic of the class. A typical feather, such as one of the long 
feathers of the tail or wing, consists of the following parts: 1. 
A horny cylindrical tube, which forms the lowest portion of the 
feather, and is termed the “quill.” 2. The “shaft,” which forms 
the central axis of the feather, and which is simply the continu- 
ation of the “quill.” The under surface of the shaft is always 
marked by a strong longitudinal groove, and it consists of a 
horny sheath, filled with a white spongy material, not unlike 
the pith of a plant. 3. The “ webs,” which form the lateral ex- 
pansions of the feather, and are attached to the sides of the 
shaft. Each web is composed of a number of small branches, 
called the “barbs,” and each barb, in turn, is furnished with a 
series of smaller fibres called the “‘barbules.” As a rule, the 
barbs are all kept in connection with one another by means of 
