Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/955

Rh TONGUE.] ANATOMY 895 Corti, and separating it from tlie endolymph of the ductus cocklearis, is the membrana tectoria, which springs from the crista spiralis close to the attachment of the membrane of Reissner, passes outwards superficial to the membrana reticularis, and ends externally at the spiral ligament. Nerve of The origin, course, and distribution of the auditory nerve hearing. j n th e labyrinth will now be considered. The auditory nerve is the portio mollis of the seventh cranial nerve. It appears at the base of the brain at the lower border of the pona Varolii. Traced to its origin its roots wind round the restiform body to the floor of the 4th ventricle, where they form the strife acousticae, and sink into the grey matter of the floor. Some of the fibres arise from an inner, others from an anterior collection of nerve cells, whilst others again are connected with the cells in the restiform body, and probably with the flocculus of the cerebellum. Where the nerve emerges at the lower border of the pons it contains a cluster of nerve cells. The auditory nerve passes down the internal meatus, and divides into a vestibular and a cochlear division. The vestibular division enters the vesti bule, and divides into five branches for the sacculus, utricu- lus, and three ampullae of the membranous semicircular canals. Each branch enters a crista acoustica and forms a plexus, in the meshes of which nerve cells are imbedded. From this plexus fine non-medullated fibres arise, which enter the layer of cells on the surface of the crista, where they anastomose and form a very delicate plexus, from which fibres spring that in all probability join the central processes of the auditory cells. The cochlear division enters a canal in the axis of the modiolus, and gives off lateral branches, which pass into the canals situated in the osseous spiral lamina. Here they radiate outwards to the membranous spiral lamina, and have connected with them collections of nerve cells forming the spiral ganglion. Beyond the ganglion they form a flat plexiform expansion, from which delicate nerves pass through a gap in the edge of the osseous lamina into the organ of Corti. In this organ the nerves, as Gottstein and Waldeyer have described, are arranged in two groups of fibres ; the inner group become continuous with the deep end of the inner hair cells ; the outer group pass across the canal of Corti and end in the outer hair cells. Hence these cells are the peripheral end-organs of the cochlear branch of the auditory nerve, or the auditory cells of the cochlea. The perilymph of the labyrinth is set in vibration by the movements of the tympanic ossicles and the fenestra ovalis; motion is thus communicated to the membranous labyrinth and the endolymph which it contains. The auditory hairs and cells would thus be set in motion, and the vestibular branches of the auditory nerve would be stimulated to con duct sound-impulses to the brain. The movements of the perilymph in the scala tympani and of the endolymph in the ductus cochlearis would set in vibration the basilar mem brane, and the auditory cells resting on it, by which the cochlear branches of the auditory nerve would be stimulated to conduct sound-impulses to the brain. It has been custom ary for physiologists to regard the vestibule as the part of the labyrinth by which sound or mere noise is determined; the cochlea, as the part which determines variations and degrees of sound, as musical notes or harmony; the semi circular canals, as determining the directions from which sound proceeds. But within the last two years experiments and arguments have been advanced almost simultaneously by Crum Brown and Mach in favour of the view that the semicircular canals act as peripheral end-organs for the sense of rotation, by which sense the axis about which rotation of the head takes place, the direction of that rotation, and its rate, are determined. Ju the account of the development of the skeleton, p. 831, it was stated that the external meatus, tympanum, Dei and Eustachian tube are the remains of the first branchial me] cleft of the embryo, that the tympanic ossicles are formed ear in the first and second visceral arches, and that the petrous bone is ossified in the cartilaginous basis cranii. The membranous labyrinth apparently arises as an invagination of the integument at the upper end of the second branchial cleft. The invaginated fold then closes in to form a shut sac, the primary auditory vesicle. Out of this vesicle the three divisions of the labyrinth are successively produced, and become enclosed by the petrous cartilage, which when ossified forms the osseous labyrinth. The epidermal invest ment of the invaginated tegumentary sac becomes trans formed into the special cell structures within the mem branous labyrinth, and the sub-epidermal connective tissue forms its fibrous walL The cochlear and vestibular nerves form at the same time as the labyrinth, and become connected through the trunk of the auditory nerve with the brain. The TONGUE, situated on the floor of the cavity of the Toi mouth, is the chief organ provided for the excitation of the special sense of taste, but the under surface of the soft palate participates to some extent in this property. The tongue is also highly endowed with the sense of touch. The structures concerned in the excitation of taste and touch are situated in the mucous membrane which envelopes the tongue. The tongue is also a muscular organ, and plays an important part in articulation, mastication, and deglutition. Its shape is flattened from above downwards, so that it presents an upper surface or dorsum and a lower surface. Its posterior part is broad, forms the base or root of the organ, and is attached to the hyoid bone. Its ante rior extremity or tip is more or less pointed, and its lateral margins or sides are rounded. The muscles connected with the tongue are arranged in pairs, and form three distinct groups, viz., accessory, extrin sic, and intrinsic muscles. The accessory muscles are the stylo-hyoid, digastric, mylo-hyoid, genio-hyoid, omo-hyoid, sterno-hyoid, and thyro-hyoid, already referred to on page 836, which act upon the hyoid bone, and thus indirectly are concerned in the movements of the tongue. The extrinsic muscles pass from adjacent parts into the substance of the tongue, arid are as follows : The stylo-glossus arises from the tip of the styloid process and the stylo-maxillary liga ment ; it runs forwards along the side of the tongue to the tip. The hyo-glossus is divided into three parts ; a, basi- glossus, which arises from the body of the hyoid ; b, cerato- glossus, from the great cornu of the hyoid ; c, chondro- glossus, from the small cornu of the hyoid. The fibres from these origins ascend into the side of the tongue. The genio-hyo-glossus arises from the upper tubercle of the symphysis of the lower jaw, its fibres radiate into the sub stance of the tongue along its whole length from base to tip ; this muscle is separated from the corresponding muscle of the opposite half of the tongue by a mesial septum of fibrous tissue. The palato-glossus arises in the substance of the soft palate, and descends to the tongue in the anterior pillar of the fauces. The intrinsic muscles lie in the sub stance of the tongue itself, and are as follows : The lingua- lis superior (noto-glossus), consisting of longitudinal fibres, which extend from the base to the tip beneath the mucous membrane of the dorsum ; the lingualis inferior, consisting of longitudinal fibres, which extend from the base to the tip along the under surface between the hyo-glossus and genio- hyo-glossus ; transverse muscular fibres, which spring from the mesial fibrous septum and curve outwards and upwards to the sides of the tongue; vertical fibres, which pass through the substance of the tongue from the dorsum to the under surface. The extrinsic and intrinsic muscles can not only