VERTEBRATE ANIMALS. 



CHAPTER XXVIII. 



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 feathers " 

 (Owen). The other leading characters which separate the Birds 

 from the other Vertubrata are, that the red blood-corpuscles are 

 nucleated ; the skull articulates with the spine by a single articulat- 

 ing surface (or condyle) ; the breathing-organs are in the form of 

 lungs, which communicate with a variable number of air - sacs 

 scattered through the body ; and the fore-limbs are never termin- 

 ated (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 (epider- 

 mis), and from their Tion-conducting nature they serve to maintain 

 the high temperatui-e of the body which is so characteristic of the 

 class. A typical feather (fig. 201), 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 continuation 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 expansions of the feather, and 

 are att.-iched 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 " bar- 

 bules." As a rule, the barbs are all kept in connection with one 

 another Ijy means of the barbules, the ends of which are hooked. 

 Towards the base of the shaft, however, the barbs are usually more 

 or less separate and placed at a distance from one another, con- 



