AVES. 287 
the shell, often by means of a temporary calcareous excrescence 
developed upon the point of the upper mandible of the bill. In 
some birds, mostly in the case of those which live upon the 
ground, the young are able to run about and look for food 
directly after they come out of the egg (Aves precoces), as is 
seen in the common Fowl. In most birds, however, the young 
are liberated from the egg in a perfectly helpless and naked 
condition, and require to be fed by their parents for a longer or 
shorter time, before they are able to take care of themselves. 
Most of these birds (Aves altrices), such as our common song- 
birds, reside in trees, and build more or less elaborate nests. 
As regards their zervous system, the brain of Birds is relatively 
larger than the brain of Reptiles, but it is destitute of those 
folds or convolutions which form so marked.a feature in the 
brain of most Mammals. The organs of sense, with the excep- 
tion of touch and taste, are well developed in Birds, vision es- 
pecially being generally extremely acute. The eyes are always 
well developed, and in no bird are they ever wanting or rudi- 
mentary, The chief peculiarity of the eye of Birds is, that its 
anterior portion (cornea) forms the segment of a much smaller 
circle than does the eyeball proper; so that the whole eye as- 
sumes a conical shape. Another peculiarity is that the form of 
the eye is maintained by means of a circle of from thirteen to 
twenty bony plates, which are placed in the front portion of the 
fibrous coat of the eye (sclerotic). Eyelashes are almost uni- 
versally absent ; but in addition to the ordinary upper and 
lower eyelids, birds possess a third membranous eyelid—the 
membrana nictttans—which is placed on the inner side of the 
eye. This nictitating membrane is sometimes transparent, 
sometimes pearly white, and it can be drawn over the front of 
the eye like a curtain, moderating the too great intensity of the 
light. As regards the organ of hearing, the chief point to re- 
mark is that Birds possess no external ear, by means of which 
the undulations of sound can be collected and transmitted to 
the internal ear. In some birds, however, as the Ostrich, the 
external opening of the organ of hearing is provided with a 
circle of feathers, which can be raised and depressed at will. 
In the nocturnal Birds, also (such as Owls), the external open- 
ing of the ear is protected by a musculo-membranous valve, 
foreshadowing the gristly external ear of Mammals. The 
sense of smell is apparently seldom very acute in Birds, and 
