The first rudiment of the internal ear is an in
volution of the integument into a small sac, which is situated
on each side of the posterior cerebral vesicle, just above the
end of the second visceral cleft. The mouth of the involution
soon closes, and a shut sac results. The sac enlarges, and, by
a remarkable series of changes, its upper part becomes (ordinarily)
converted into three
semicircular canals - the anterior and
posterior vertical, and the
external or
horizontal canals of the
membranous labyrinth. The body of the sac remains,
lor the most part, as the
vestibule; but a caecal process, which
eventually becomes shut off from the vestibule, is given off
downward and inward, toward the base of the skull, and is
the rudiment of the
scala media of the
cochlea. This may be
called the
membranous cochlea.
In the anomalous vertebrate, Amphioxus, no ear has yet
been discovered. The Hag (
Myxine) has only one, and in the
Lampreys (
Petromyzon) there are only two, semicircular canals; but, in fishes in general, all three are developed, and it
is a question whether the cochlea is not also represented.
In fishes, the periotic cartilage and its ossifications enclose
this membranous labyrinth, externally, and present no merely
membranous gaps, or
fenestrae, toward the first visceral cleft,
or the space which represents it.
But in higher Vertebrata (
Amphibia, Sauropsida, Mammalia), in which the membranous labyrinth is always enclosed
within a complete bony periotic capsule, the outer wall of
this capsule invariably remains unossified over one or two
small oval areas, which consequently appear like windows with
membranous panes, and are termed the
fenestra ovalis and the
fenestra rotunda.
The
fenestra ovalis is situated in that part of the periotic
mass which bounds the chamber containing the membranous
vestibule externally; and it is always found that, when both the
prootic and the opisthotic bones exist, they contribute nearly
equal shares to the formation of its boundaries. In fact, the
fenestra ovalis is situated in the line of junction of these two
bones. The
fenestra rotunda, on the other hand, is below
the
fenestra ovalis, and lies altogether in the opisthotic. It
forms part of the outer wall of the cavity in which the membranous
cochlea is lodged.
In the
Sauropsida and
Mammalia, this membranous cochlea,
become flattened and bandlike, and its communication
with the vestibule obliterated, is lodged in a conical cavity, in
such a manner as to divide that cavity into two portions,
called
scalce, which only communicate at their apices. The
base of the one scala, called
scala vestibuli, opens into the
cavity which contains the membranous vestibule: that of the
other,
scala tympani, abuts against, and is as it were stopped
by, the membrane of the fenestra rotunda. The cavity of the
membranous cochlea stretched between, and helping to divide,
these two soalce, is called the scala media.
In Reptiles, Birds, and Ornithodelphous Mammals, the
cochlea is only slightly bent or twisted upon itself. But, in
the higher
Mammalia, it becomes coiled in a flat or eonical
spiral of one and a half (
Cetacae, Erinaceus) to five (
Coelogenys
Paca) turns.
The membranous labyrinth is filled with a clear fluid, the
endolymph, and usually contains otolithes of various kinds.
Between the membranous labyrinth and the walls of the cavity
of the periotic mass in which it is contained, lies another
clear fluid, the
perilymph, which extends thence into the
scaloe
vestibuli and
tympani.
In all animals which possess a
fenestra ovalis, its membrane
gives attachment to a disk, whence an ossified rod, or
arch, proceeds. Where the former structure obtains, as in
Birds, most Reptiles, and some
Amphibia, the bone is commonly
called
columella auris; when the latter, as in most
Mammals,
stapes. But there is really no difference of importance
between
stapes and
columella, and it is advisable to use
the former name for the bone under all its forms.
In the majority of
Vertebrata of higher organization than
fishes, the first visceral cleft does not become wholly obliterated,
but its upper part remains as a transversely elongated
cavity, by means of which the pharynx would be placed in
communication with the exterior, were it not that the opposite
sides of the canal grow together into a membranous partition -
the
membrana tympani. So much of the canal as lies
external to this is the external
auditory meatus; while what
lies internal to it, is the
tympanum, or drum of the ear, and
the
Eustachian tube, which places the tympanum in communication
with the pharynx. While the outer wall of the tympanum
is the tympanic membrane, its inner wall is the periotic
mass with its fenestroe, and, in all
Vertebrata below Mammals,
the outer end of the
stapes is either free, or, more commonly,
is fixed to the tympanic membrane, and thus the latter
and the membrane of the
fenestra ovalis become mechanically
connected. In all these animals the mandible is connected
with the skull by the intermediation of an as
quadratum.
But, In the
Mammalia, the mandible is articulated directly
with the squamosal, and the
quadratum is converted into one
of the so-called
ossicula auditus, and named the
malleus. The
malleus becomes attached to the
membrana tympani, by a
special process; while its other extremity, which was continuous
with Meckel's cartilage in the embryo, is converted into
the
processus gracilis, or
Folianus, and lies between the tympanic,
the squamosal, and the periotic bones.
 |
Fig. 24. - Diagram of the skeleton of the first and second visceral
arches
in a Lizard (A)
Mammal (B), and an Osseous Fish (C).
The skeletol of the
first visceral arch is shaded, that
of the second
is left nearly unabaded, I. First visceral arch. Mck. Meckel's cartilage. Art, Articulare. Qu. Quadratum. Mpt. Metapterygoid; M. Malleus; p.g., Processus gracilia II. Second visceral arch Hy. Hyoidean corna. St. II.
Stylohyal. S. Stapedius. Stp. Stapes. S. Stp Suprastapedial. HM.
Hyomandibular. The arrow
indicates the first visceral
cleft. Ft The
periotic capsule. Ptg. The
pterygoid. |
In the singular lizard
Sphenodon (A, Fig. 24), the anterior
cornu of the hyoid is continuous with the distal end of the
stapes, and the latter sends a cartilaginous process upward,
which passes into the wall of the periotic capsule, just behind
the proximal end of the
os quadratum. Thus the stapes
stands out at right angles to the hyoid cornu, and the latter
becomes divisible into a
supra - stapedial part, and a part which
lies below the stapes, and answers to the styloid process, or
stylohyal, of the
Mammalia. The supra - stapedial part is represented
by cartilage, or ligament, in other
Sauropsida, but
seems not to ossify. In the
Mammalia (B, Fig. 24), the supra -
stapedial part ossifies, becomes the
incus, and its proximal
end is usually articulated by a synovial joint with the
malleus ( =
quadratum). A distinct ossification, the
os orbiculare,
usually arises at that part of the hyoidean cartilage in which
the
stapes and the
incus unite. That part of the hyoidean
cartilage which is converted into the styloid process is generally
connected with the
orbiculare by muscular fibres, which
constitute the
stapedius muscle. On the other hand, the posterror,
or short process of the
incus, is connected by ligament
with that part of the periotic mass into which the styloid process
is directly continued, and it is hard to say whether the
styloid part of the hyoid is continued into the incus by these
ligaments or by the
stapedius. But, however this may be, the
malleus and the incus are the proximal ends of the mandibular
and hyoidean arches respectively.
In osseous fishes (C, Fig. 24), which have no fenestra ovalis
or stapes, the supra-stapedial part of the hyoid becomes a
large bone - the
hyomandibular. On the other hand, the
proximal extremity of the quadrate cartilage atrophies, loses
its direct connection with the periotic capsule, and becomes
distinctly ossified, as the
metapterygoid. In the Sharks, even
the ascending, rnetapterygoid, part of the quadrate, is lost.
The quadrate and supra - stapedial portions of the first and
second visceral arches coalesce in the
Chimaera,
Dipnoi, and
many
Amphibia, into a single cartilaginous plate.
In the
Mammalia, and to some extent in
Aves, osseous
matter is deposited in the fibrous tissue which surrounds the
sides and base of the tympanic membrane, and gives rise to a
special
tympanic bone. In most
Mammalia, ossification extends
into the sides and floor of the tympanum and external
meatus; and a process of integument, chiefly derived from the
second visceral arch, is converted into a
concha, or
external ear.
The
Organ of Taste is the mucous membrane which covers
the tongue, especially its posterior region, and probably also
a part of that lining the fauces. When the sense is well developed,
the mucous membrane is raised into numerous papillae
of various forms, and is well supplied with filaments from the
glossopharyngeal nerve.
The sense of
Touch is diffused over the integument and
over the mucous membrane of the buccal cavity, which is,
strictly speaking, a part of the integument.
As special organs of touch in the higher
Vertebrata, the
nervous papillae, containing
"tactile corpuscles," and the long
facial hairs, the papillae of which are well supplied with nerves,
termed
vibrissae, may be mentioned.
In most, if not all Fishes, the integument of the body and
of the head contains a series of sacs, or canals, usually disposed
symmetrically on each side of the middle line, and filled with
a clear gelatinous substance. The walls of the sacs, or canals,
are abundantly supplied with nerves, and the terminations of
the latter enter rounded
papillae, which project into the gelatinous
contents. These sensory organs are known as the "
organs of the lateral line" or
"mucous canals;" and they
were formerly supposed to be the secretory glands of the
slimy matter which coats the bodies of fishes, and which is
really modified epidermis.