Auditory Paths.—There are two auditory paths, cochlear and the vestibular. The former is concerned with hearing and the latter with equilibrium.
i. The Cochlear Path (Figs.74, 93 and 9).—Impulses of hear ing originate in the organ of Corti. • They are transmitted by the rods and hair cells of Corti to the dendrites of the spiral gan glion. Traversing the dendrites and cell-bodies of that ganglion they enter the axones which form the cochlear nerve and run backward to the terminal nucleus of that nerve in the medulla. Both the ventral and the lateral portions of the cochlear nucleus receive the impulses of hearing. From the cochlear nucleus they run either lateral and dorsal to the restiform body and cross to the opposite side through the medullary striae and trapezoid body, or they run medial to the restiform body and enter at once into the trapezoid body. By either course they reach the lateral fillet and chiefly the opposite one. The lateral fillets conduct the impulses to the inferior quadrigeminal colliculi; the brachia inferiora to the medial geniculate bodies, and the thalamo temporal radiations to the third and fourth fifths of the superior temporal and to the transverse temporal gyri of the cerebrum. Through the lateral fillet, impulses producing reflex reach the quadrigeminal colliculi and thence by the anterior tecto spinal bundle pass to motor nuclei and also through the oli vary pedicle and medial longitudinal bundle they reach the nu clei of the sixth, fourth and third cerebral nerves.
2. Vestibular Path.—The extent of the vestibular conduction path is from the acustic areas of the utricle, saccule and semi circular canals to the vestibular nuclei in the floor of the fourth ventricle; and thence to the cerebellum and to the cortical area of equilibrium, according to Mills, in the temporal cortex. It is the path of space sense. Through the vestibular nerve the im pulses reach the dorso-medial, the dorso-lateral and superior nucleus, and the nucleus of the descending root in the floor of the fourth ventricle.
The impulses may pursue, from the terminal nuclei in the ventricular floor, either a direct or an indirect course to the cerebral cortex.
1. By the direct course they run through the opposite medial fillet and certain fibers in the cortical fillet, perhaps the ventral stalk of the thalamus, to the middle and inferior temporal gyri.
2. The impulses run to the cerebellum, by the indirect course, through the fibers of the vestibular nerve that run without interruption to nucleus fastigii and the nucleo-cerebellar fibers which run from each of the vestibular nuclei to the cerebellar cortex. Both sets of fibers pass through the restiform body, but only the latter reaches the cortex. They excite in the cere bellum impulses of equilibrium and then continue upward. From the cerebellum the course of the impulses is, presumably, through the brachium conjunctivum to the red nucleus and thala mus of both sides and thence to the cortex.
Impulses concerned with reflexes run from the vestibular nuclei in the floor of the fourth ventricle, (a) to the opposite nuclei of motor cerebral nerves via the medial longitudinal bun dle; (b) to the quadrigeminal colliculi through the superior fillet; (c) to the motor nuclei of spinal nerves through the vestibulo spinal tract; and from the cerebellar cortex the impulses reach the motor nuclei of both cranial and spinal nerves, as follows: through the cortico-nuclear neurones to the cerebellar nuclei; the cerebello-tegmental tracts from those nuclei, through the brachium conjunctivum and restiform body, to the brain-stem; and then complete their journey through the vestibulo-spinal, rubro-spinal, and thalamo-spinal tracts.
The Gustatory extend from the tongue to the nucleus tractus solitarii in the medulla and thence probably through the opposite formatio reticularis and internal capsule to the taste area in the gyrus cinguli (Flechsig). There are two paths from the tongue to the nucleus of the solitary tract. Those impulses from the base of the tongue and the palate run through the ninth nerve and those from the anterior two-thirds of the tongue through the chorda tympani and intermediate nerve to the medulla (A. F. Dixon, Keen and Spiller, H. Cushing, etc.). Possibly, gustatory impulses originating in the palate may tra verse the descending palatine nerves and the great superficial petrosal nerve to reach the geniculate ganglion on the facial and then continue through the intermediate nerve to the solitary tract. All impulses arriving at this nucleus of the solitary tract probably complete their journey in two stages: First, through the formatio reticularis to the opposite thalamus, and second, through internal capsule to thecortex. May and Horsley have traced the gustatory tract from the nucleus of the solitary tract upward through the reticular formation close to the cen tral gray substance and dorso-lateral to the medial longitudinal bundle, to the lateral nucleus of the thalamus; the tract enters the internal medullary lamina of the thalamus and terminates in the medial part of the dorsal third of the great lateral nucleus (Brain, Vol. 33). The position of the gustatory radiation in the internal capsule is not yet determined; between the optic radiation and the parietal stalk of the thalamus is the most probable location.
Destruction of the olfactory conduction path on one side causes anosmia on the same side; of the optic tract or radiation, atrophy and destruction in the corresponding halves of both retinae; reflexes are abolished in the affected area in the first case, but preserved for a time when the lesion is in the optic radiation; interruption of the auditory path above the pons, deafness chiefly on the opposite side and interruption of the gustatory path above the medulla oblongata abolishes taste on the same side.