278 VERTEBliATE ANIMAL!?. 



CTIAl'TKi; XXYIII. 

 ('LASS IV. AYES. 



The fourth class of tlie Verteln-ates i.s that of the Birds or Ares, 

 ■\vhich mjy be shortly detiutid as being " oviparous Vertebrates, 

 with warm blood, a double cinadatiou, and a covering of featliers" 

 ((hvenj. Tlie other leading characters which separate the Birds 

 from the other VrrtchrKtn. are, that the red blood-corpuscles are 

 nucleated; the ,sladl articulates with the spine by a single articulat- 

 ing surface (or ci>ndylci; the breathing-organs are in the form of 

 lungs, whicli connnunirate with a variable ijiuuber of air - sacs 

 scattered through the body; and the bjre-lirnljs are never termin- 

 ated (in existing birds) by mure than two lingers, ending in claws, 

 and ai'e generally rnoditied so as to form "wings" or organs of 

 flight. 



The feathers, which for]u such a distinctive character of birds, 

 are fcjrmed liy a moililicatiou of the outer layer of the skin (epider- 

 mis), and from their non-ci>nducting nature they serve to maintain 

 the high tcmjieratnre of the boily which is so characteristic of the 

 cla.ss. A typical feather (iig. :iOI), such as one of the long feathers 

 of the tail or wing, consists of the fcillowing jiarts : 1. A horny 

 c\linilrical tube, wliicli forms the lowest portion of the feather, and 

 is teruied the "i|uil]." 2. Tlic "shaft," which forms the central 

 a.\is iif the fi'allici', and which is sim]>ly the contimiation of the 

 "i|iiill." 'J'lic undi'r sui-face of the shaft is always marked by a 

 string longitudinal gi'oovc, and it ci nsi^ts i^f a horny sheath, tilled 

 with a v\]iite spongy matei'ia], not unlike the jiitli of a phuit. 3. 

 The "webs," which form the lateial ex]iansions cjf the feather, and 

 are attached to the sides of the shaft. Each wvh is comp(].sed of 

 a number of small bi'anches, called tlie "barbs"; and each barb, 

 in turn, is furnished with a series of smaller fibres called the "bar- 

 bule.s." As a rule, the barbs are all kejit in connection with one 

 another by means of the barbules, the ends of whii'h are hooked. 

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

 01' less sej.arnte and jilaced at a di-tance from one another, con- 



