EYE AND EAR. 
SI 
the cerebellum, though never wanting, consists simply of the central 
lobe, and lacks the lateral lobes which occur in the mammals, or 
they are present only in a rudimentary form. 
In connection with the intelligence of the bird must be considered 
his organs of sense; for when these are defective or imperfect, the 
intelligence is invariably defective or imperfect. " The eyes are always 
well developed, and in no bird are they ever rudimentary or absent." 
Such is Professor Nicholson's dictum; and he proceeds to point out 
some peculiarities of the bird's eye. For example, the cornea forms a 
segment of a much smaller sphere than does the eyeball proper ; so that 
while the anterior part of the eye is obtusely conical, the posterior 
portion is spheroidal. Again, the form of the eye is maintained by a 
circle of from thirteen to twenty bony plates, situated in the anterior 
portion of what is called the sclerotic coat. Few birds have eyelashes ; 
but, in addition to the ordinary upper and lower eyelid, they possess 
a third membraneous eyelid, the membrana nictitans, — -in man only 
rudimentary, — which is sometimes pearly-white, sometimes more or 
less transparent. This winldng membrane (to speak literally) is placed 
on the inner side of the eye, and, by means of a special muscular 
apparatus, falls over the anterior surface of the eye like a veil, 
subduing the intensity of the external light. 
Most birds have no outer ear, or concha, by which sounds can be 
gathered up and conveyed to the inner ear. In some species, however, 
as in the ostrich and bustard, the external meatus auditorius is 
surrounded by a circle of feathers, which can be raised or depressed at 
will. That the hearing of birds is admirable, is proved by the excel- 
lence of their song: for song is not with them, as is popularly supposed, 
an innate or inherited faculty — a gift from Heaven ; it has to be 
acquired. The fledgelings learn by listening to and imitating their 
mother. Frequently, it is through their keen hearing that they are 
warned of the approach of danger. They enjoy one another's com- 
panionship by means of the same quickness of sense which enables 
them to recognize and comprehend the lightest sounds. It is a curious 
fact, however, that the song-birds which are sensible to music are those 
whose hearing is the most imperfect ; while birds with a delicate ear, 
