476 HISTOLOGY 



NERVES OF THE LABYRINTH. 



The acoustic nerve is a purely sensory nerve passing between the pons 

 and internal ear through a bony canal, the internal acoustic meatus. It is 

 divided into vestibular and cochlear portions (Fig. 474). The vestibular 

 nerve proceeds from the vestibular ganglion and has four branches the 

 utricular nerve and the superior, lateral, and posterior ampullary nerves; 

 according to Streeter (Amer. Journ. Anat., 1906, vol. 6, pp. 139-165) it 

 produces also the branch to the sacculus, usually regarded as derived 

 from the cochlear nerve. If this is true, the cochlear nerve supplies 

 only the spiral organ of Corti. The ganglion of the cochlear nerve is 

 lodged within the modiolus at the root of the lamina spiralis, and is known 

 as the spiral ganglion (Figs. 474 and 477). The ganglion cells remain 

 bipolar, like those of embryonic spinal ganglia. They are surrounded by 

 connective tissue capsules; and their neuraxons and single peripheral 

 dendrites receive myelin sheaths not far from the cell bodies. 



The peripheral fibers extend through the lamina spiralis ossea, within 

 which they form a wide-meshed plexus, and after losing their myelin they 

 emerge from its outer border in the labium tympanicum through the 

 foramina nervosa. In continuing to the spiral organ they curve in the 

 direction of the cochlear windings, thus producing spiral strands. Those 

 nearest the modiolus are on the axial side of the pillar cells; the middle 

 ones are between the pillars, in the tunnel ; and the outer ones are beyond 

 the pillar cells. From these bundles, delicate fibers pass to the hair cells, 

 on the sides of which they terminate. 



VESSELS OF THE LABYRINTH. 



The. internal auditory artery is a branch of the basilar artery. It 

 arises in connection with branches which are distributed to the under 

 side of the cerebellum and the neighboring cerebral nerves, and passes 

 through the internal acoustic meatus to the ear. It divides into vestibular 

 and cochlear branches (Fig. 481). The vestibular artery supplies the 

 vestibular nerve and the upper lateral portion of the sacculus, utriculus 

 and semicircular ducts. The cochlear artery sends a vestibulo-cochlear 

 branch to the lower and medial portion of the sacculus, utriculus, and ducts. 

 This branch also supplies the first third of the first turn of the cochlea. 

 The capillaries formed by the vestibular branches are generally wide 

 meshed, but near the maculae and cristae the meshes are narrower. The 

 terminal portion of the cochlear artery enters the modiolus and forms three 

 or four spirally ascending branches. These give rise to about thirty radial 

 branches distributed to three sets of capillaries (Fig. 482); i, those to the 

 spiral ganglion; 2, those to the lamina spiralis; and 3, those to the outer 

 walls of the scalae and the stria vascularis of the cochlear duct. 



