288 VERTEBUATE ANIMALS. 



are liljeratcil from tlie egg in ii perfectly helpless and naked con- 

 dition, and reipiireto lie fed liy their parents for a longer or shorter 

 time, before they are aljle to take care of themselves. Most of 

 these Birds (Jcc.s- fdtrire.<:), .such as our common Soug-birds, reside iu 

 trees, and l.iuild more or less el;il)orati' nests. 



As regards their iii-riYiiis .ii/stem, the lir:iin of Birds is relatively 

 larger than the l)r,'iin of Rej)tiles, but it is destitute of those folds or 

 convolutions which f^rm so marked a feature in the Ijrain of most 

 Mannuals. The oig:ins of sense, with the exception of touch and 

 taste, are well develo|jed in Birds, vision especially being generally 

 extremely acute. The eyes are always well developed, and in no 

 Bird are they ever wanting or rudimentary. The chief peculiarity 

 of the eye of Birds i.s, that its anterior portion {corned) forms the 

 segment of a nuich sm.-dler circle than does the eyeball proper ; so 

 that the ^Yllole eye assumes a conical sh.ajje. Another peculiarity is, 

 that the form of the eye is maintained by means of a circle of from 

 thirteen to twi'uty liony plates, which are placeii in the front portion 

 of the fibrous coat of the eye (xi-hji-dtir). Eyelashes are almost imi- 

 versally absent ; Ijut in addition to the ordinary ujiper and lower 

 eyeliils, Bir.ls jmssess a third meudiranou.s eyelid — the niemhrana 

 idrti/ii/is — wliiili is ]ilaced on the inner side of the eye. This nictitat- 

 iuLj nienibraiie is sometimes transparent, scjmetimes pearly white, 

 and it can be drawji over the front of the eye like a curtain, mod- 

 er:itiiig the too great intensity of the light. As regards the organ 

 of hearing, th(; chief |ioiut to remark is, that Birds posness no external 

 ear, by means of wliirh the undulations of sound can be collected 

 and transmitted to the internal e.nr. In .some Birds, however, as the 

 Cstrich, the external ojiening (jf the organ of hearing is provided 

 Avith a circle of feathers, which cm l)e rai.sed and depressed at will. 

 In the norturn.al Birds, also (such as ()\vls), the external ojiening of 

 the ear is protected by a mus(ido-membranous valve, foreshadowing 

 the gristly external ear of Mannuals. The sense of smell is ajipa- 

 rently .seldom very acute in Birds ; and even the Birds of Prey appear 

 to seek their food m.aiidy by the sight. The external nostrils are 

 usuiilly ]il.'iced ou the sides of the upper mandible, near its base, and 

 form simjile ]ierforations wdiich sometimes communicate from side 

 t ) side. In the curioirs A/iferi/.r of New Zealand, the nostrils are 

 |ilaceil nt the extreme end of the elongated be.ik. Sometimes the 

 nosti'ils are defended by bristles, and sometimes by a cartilaginous 

 sc:de. 



Before passing on to a co)isideratiou of the diyisions of Birds, a few 

 woi-ds m.'i.y be sidil on the /»/r/;vrf/ij«,s- of Bird.s. In temperate and 

 i;nl,\ I lihi.itcs, only certain birds remain constantly in the same region 

 in \vIm( II thiy were originally hatched. Those which do so are called 



