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  Section: Anatomy of Vertebrate Animals » The Muscles and the Viscera
 
 
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The Muscles of the Ear

 
     
 

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.
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.
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.

 
     
 
 
     



     
 
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