The _Eye_ is essentially reptilian, but in sharpness of vision, power
and quickness of accommodation it surpasses that of the mammals. The
eyeball, instead of being globular, resembles rather the tube of a short
and thick opera-glass.
The anterior half of the sclerotic is composed of a ring of some ten
to seventeen cartilaginous or bony scales which partly overlap each
other. Another cartilage or ossification, the posterior sclerotic
ring, occurs within the walls of the posterior portion of the cup, and
surrounds, especially in the Pici and in the Passeres, the entrance of
the optic nerve. The iris is in most young birds at first brown or
dull-coloured, but with maturity attains often very bright tints which
add considerably to the charm of the bird; sexual dimorphism is in
this respect of common occurrence. The iris contains a sphincter and a
dilator muscle; the former, supplied by branches from the
_oculomotorius_ nerve, is under control of the will, whilst the
dilator fibres belong to the sympathetic system. When fully dilated,
the pupil is round in all birds; when contracted it is usually round,
rarely oval as in the fowl. From near the entrance of the optic nerve,
through the original choroidal fissure, arises the much-folded pecten,
deeply pigmented and very vascular, far into the vitreous humour. The
number of its folds varies considerably, from three in _Caprimulgus_
to nearly thirty in crow (_Corvus_). _Apteryx_, which since Owen has
generally been stated to be devoid of such an organ, likewise
possesses a pecten; its base is, however, trumpet-shaped, covers
almost the whole of the optic disk, and extends nearly to the lens in
the shape of a thick, densely pigmented cone, without any plications,
resembling in these respects the pecten of many Lacertilia (see G.L.
Johnson, _Phil. Trans._, 1901, p. 54). In the retina the cones prevail
in numbers over the rods as in the mammals, and their tips contain, as
in other Sauropsida, coloured drops of oil, mostly red or yellow. Near
the posterior pole of the fundus, but somewhat excentrically placed
towards the temporal or outer side, is the _fovea centralis_, a slight
depression in the retina, composed almost entirely of cones, the spot
of most acute vision. Many birds possess besides this temporal fovea a
second fovea nearer the nasal side. It is supposed that the latter
serves monocular, the other the binocular vision, most birds being
able to converge their eyes upon one spot. Consequently the whole
field of vision of these birds possesses three points where vision is
most acute. It may here be remembered that of the mammalia man and
monkeys alone are capable of convergence, and have a circumscribed
macular area.
Of the outer eyelids, the lower alone is movable in most birds, as in
reptiles, and it frequently contains a rather large saucer-shaped
cartilage, the _tarsus palpebralis_. The margins of the lids are
sometimes furnished with eyelashes, e.g. in the ostrich and in the
Amazon parrots, which are vestigial feathers without barbs. During the
embryonic stage the lids are fused together, and either become
separated shortly before the bird is hatched, as is the case with most
Nidifugae, or else the blind condition prevails for some time, in the
young Nidicolae. All birds have, like most reptiles, a well-developed
third lid or "nictitating membrane," which moves from the inner
canthus obliquely upwards and backwards over the cornea. The moving
mechanism is a further and much higher development of that which
prevails in reptiles, there being two muscles completely separate from
each other. Both are supplied by the _abducens_ nerve, together with
the _rectus externus_ muscle. One, the _quadratus_ or _bursalis_
muscle, arises from the hinder surface of the eyeball, and forms with
its narrow margin, which is directed towards the optic nerve, a pulley
for the long tendon of the _pyramidalis_ muscle. This arises from the
nasal surface of the ball, and its tendon passes into the somewhat
imperfectly transparent nictitating membrane. The quadrate muscle
adjusts the motion, and prevents pressure upon the optic nerve; during
the state of relaxation of both muscles the nictitans withdraws
through its own elasticity.
See R. Leuckart in Graefe and Saemisch's _Handbuch d. Ophthalmologie_
(Leipzig, 1876, vol. i. chap. 7); H. Mueller, _Gesammelte Schriften_
(Otto Becker, Leipzig, 1872), and _Arch. f. Ophthalmol._ iii.; Ch.
Rouget, "Recherches anatomiques et physiologiques sur les appareils
erectiles," "Appareil de l'adaptation de l'oeil" ... _Compt. Rend._
(Paris, xlii., 1856, pp. 937-941); M. Schultze, art. "Retina," in
Stricker's _Handbuch der Gewebelehre_, 1871, vol. ii.; J.R. Slonaker,
"Comp. Study of the Area of Acute Vision in Vertebrates," _Journ.
Morph._, 1897.
_Ear._--The outer opening of the ear is, with rare exceptions, concealed
by feathers, which are often rather stiff, or modified into bristles.
There is no other protection, but slight, imperfectly movable folds of
skin arise from the outer rim. The largest ear-opening is met with in
the owls, with correspondingly larger folds of skin, the function of
which is less that of protection than, probably, the catching of sound.
In many owls the right and left ears are asymmetrical, and this
asymmetry affects the whole of the temporal region, all the bones which
surround the outer and middle ear, notably the squamosal and the
quadrate, so that the skull becomes lopsided, one ear being turned
obliquely down, the other upwards. (For, detail see Collett,
_Christiania Vidensk. Forhandl._, 1881, No. 3.)
[Illustration: FIG. 16.--Auditory "chain" of Chicken. Lateral and basal
views. (After W.K. Parker).]
The middle ear communicates with the mouth by the Eustachian tubes,
which pass between the basisphenoid and basioccipital bones, and unite
upon the ventral side of the sphenoid, a little behind its
articulation with the pterygoids, where they open into the mouth
cavity by a short membranous duct. The columellar apparatus, or
auditory chain of ossicles (fig. 16), extending between the fenestra
ovalis and the tympanic membrane or drum, consists of (1) the long and
slender columella, a straight, ossified rod which fits with a disk
into the fenestra ovalis; it is homologous with the stapes (_m.st._),
although not stirrup-shaped; (2) the extracolumellar mass. This is
chiefly cartilaginous and sends out three processes: the dorsal (_s.
st._) is attached to the upper wall of the drum cavity; the outermost
(_e. st._) is fastened on to the middle of the drum membrane; the
third, ventral or infracolumellar process (_i. st._) is directed
downwards and tapers out into a thin, partly cartilaginous, strand,
which originally extended to the inner corner of the articular portion
of the mandible, but on its long way comes to grief, being squeezed in
between the pterygoid and quadrate. This long downward process being
homologous with an almost exactly identical arrangement in the
crocodile, and with the _processus folii_ of the mammalian malleus, it
follows that the whole extracolumellar mass, that between stapes and
drum, is equivalent to incus and malleus of the mammalia. There is, in
birds, no _annulus tympanicus_. Birds possess an ear-muscle which at
least acts as a _tensor tympani_; it arises near the occipital
condyle, passes through a hole into the tympanic cavity, and its
tendon is, in various ways, attached to the inside of the membrane and
the neighbouring extracolumellar processes.
As regards the inner ear, the endolymphatic duct ends in a closed
_saccus_, imbedded in the dura mater of the cranial cavity. The apex
of the cochlea is turned towards, and almost reaches the anterior wall
of the occipital condyle; at most it makes but half a twist or turn;
it possesses both Reissner's membrane and the organ of Corti. Although
the _scala tympani_ is so rudimentary, not reaching a higher level
than in most of the reptiles, and remaining far below the mammalia,
birds do not only hear extremely well, but they distinguish between
and "understand" pitch, notes and melodies.
See G. Breschet, _Recherches anatomiques et physiologiques sur
l'organe de l'audition chez les oiseaux_ (Paris, 1836), with Atlas; C.
Hasse, various papers in _Zeitschr. f. wiss. Zool_. vol. xvii, and in
_Anatomische Studien_, pts. ii. and iv. (Bresku, 1871); I. Ibsen,
_Atlas anatomicus auris internae_ (Copenhagen, 1846); G. Retzius, _Das
Gehororgan der Wirbelthiere_ (Stockholm, 1884), ii. pp. 139-198, pls.
15-20.
_Nose._--The olfactory organ is poorly developed, and it is still a
question whether birds possess much power of smell; many are certainly
devoid of it.
The olfactory perceptive membrane is restricted to the posterior
innermost region of the nasal chamber, where it covers a slight
bulging-out prominence on the nasal wall. This so-called third, upper
or posterior conch is not a true conch, nor is that of the vestibulum;
only the middle one forms a scroll, and this corresponds to the only
one of reptiles and the lower of the mammals. The nasal cavity
communicates with the mouth by the choanae or posterior nares,
situated between the palatine process of the maxillary, the palatine
and the vomer. The outer nares or nostrils are most variable in size
and shape. In the Steganopodes they tend to become much reduced, e.g.
in cormorants (_Phalacrocoracidae_), and especially in _Sula_, where
the nasal slits become completely closed up, and the greater portion
of the nasal cavity is also abolished, being restricted to the
olfactory region with its unusually wide choanae. The nasal septum is
often more or less incomplete, producing _nares peniae_, e.g. in the
Cathartae, in the Anseres, gulls, rails and various other aquatic
birds. The secretions of the mucous membrane of the nasal cavity, and
a pair of naso-lacrymal glands (not to be confounded with the
Harderian and the lacrymal glands), moisten and clean the chamber. The
glands are variable in size and position; when very large, e.g. in
plovers, they extend upon the forehead, causing deep impressions on
the bones of the skull. Jacobson's organ has been lost by the birds,
apparently without a trace in the embryonic fowl, but T.J. Parker has
described vestiges of the corresponding cartilages in the _Apteryx_
(_Phil. Trans._, 1890).
See C. Gegenbaur, "Ueber die Nasenmuscheln der Voegel," _Jena Zeitschr_.
vii., 1873, pp. 1-21.