CHAPTER XXIX. 



CLASS IV.— AVES. 



The fourth class of the Vertebrates is that of the Birds or 

 Aves, which may be shortly defined as being " oviparous Ver- 

 tebrates, with warm blood, a double circulation, and a cover- 

 ing of feathers " (Owen). The other leading characters which 

 separate the Birds from the other Vertebrata are that the red 

 blood-corpuscles are nucleated, the skull articulates with the 

 spine by a single articulating surface (or condyle), the breath- 

 ing-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 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 (epidermis), and from their non-conducting nature they 

 serve to maintain the high temperature of the body which is 

 so characteristic of the class. A typical feather, such as one 

 of the long feathers of the tail or wing, consists of the follow- 

 ing 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 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 " bar- 



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

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