The ventral cochlear nucleus appears in section as a triangular mass of cell-bodies imbedded in the medulla at the upper end of the posterior lateral sulcus. It lies between the restiform body and the olive; the vestibular root of the auditory nerve separates it from the olive. It receives the greater number of fibers in the cochlear nerve and gives rise to those of the trape zoid body and through that to a large part of the lateral fillet of the opposite side; a few of its fibers enter the fillet of the same side. In the corpus trapezoideum, the cochlear tract is largely relayed by the neurones forming the nuclei of the superior olivary group. The lateral cochlear nucleus embraces the outer surface of the restiform body. It is situated both lateral and dorsal to the ventral nucleus and, stretching around the posterior surface of the restiform body, it produces the ventricu lar eminence in the lateral part of the acustic area, called the tuberculum acusticum. The lateral nucleus receives that part of the cochlear root which does not end in the ventral nucleus and the fibers arborize about its cells. The axones of the lateral nucleus form the medullary striae; a few of them enter the trapezoid body (Figs. 112, Ii9 and 123 ). The medul lary striae run somewhat obliquely across the ventricular floor to the median groove, plunge forward to the superior olivary nucleus of the opposite side where they are partially relayed and then. bending upward, are continued in the lateral fillet. At the superior olivary nuclei of the opposite side the fibers from the lateral and ventral nuclei become intermingled, hence the trapezoid body and medullary stria: combine in the formation of the lateral fillet. The lateral fillet suffers a partial relay in its own nucleus, after which it separates into two parts; the prin cipal part runs to the medial geniculate body by way of the brachium inferius; the smaller part ends in the quadrigeminal colliculi, chiefly in the inferior colliculus on the same side. From the medial geniculate body to the transverse and superior temporal gyri the acustic path is formed by the acustic radia tion. This completes the cortical connection of the cochlear nuclei. Their reflex connections are established, first, by the olivary pedicle and medial longitudinal bundle and, second, by that part of the lateral fillet which ends in the colliculi of the corpora quadrigemina and the tecto-spinal tracts (see pp. 155 and 164).
3. There are Certain Special Nuclei of the Medulla.— These are not represented either in the pons above or the spinal cord below. They are the nucleus funiculi gracilis, the nucleus funiculi cuneati and the nucleus olivaris inferior.
Nucleus Funiculi Gracilis and Nucleus Funiculi Cuneati (Figs. 125 and 126).—The nucleus funiculi gracilis and nucleus funiculi cuneati are large nuclei, extending from the level of the olive to the lower end of the medulla. They are situated near the posterior surface beneath the gracile and cuneate funiculi, whose fibers terminate in them; they give origin to the medial fillet, and the anterior and posterior external arcuate fibers, and they produce, respectively, the clava and cuneale tubercle on the posterior surface of the medulla. In successive sections from below upward the nucleus funiculi gracilis is first seen as an isolated mass of gray substance imbedded in the funiculus gracilis at the level of the pyramidal decussation. It enlarges dorso-ventrally and transversely toward its upper end, as is shown in consecutive sections and reaches its greatest size at the clava where it receives the terminal end-tufts of the funiculus gracilis. Very soon the ventral border of the nucleus funiculi gracilis fuses with the gray matter about the central canal. The axones of this nucleus form about one-half of the medial fillet and the external arcuate fibers. The nucleus funiculi cuneati (Fig. 126) appears at the same inferior level as the nucleus funiculi gracilis. It is from the first and throughout its length continuous with the central gray substance on which it appears as a bud-like outgrowth in the lower medulla. It gradually broadens and elongates dorsalward when traced up ward (Fig. 125). Beneath the cuneate tubercle it reaches its full stature and gathers into itself the fibers of the funiculus cunea tus; thence it sends its own axones upward in the medial fillet and the external arcuate fibers. Near the lower end of the medulla there is a small lateral bud of gray matter connected with the nucleus funiculi cuneati to which it is accessory and, like it, is imbedded in the funiculus cuneatus. It is called the accessory nucleus funiculi cuneati.
With entire accuracy and greater convenience, these nuclei may be called nucleus gracilis and nucleus cuneatus. They are somatic terminal nuclei of spinal nerves.
The nuclei gracilis et cuneatus form the first relay station in the spino-cerebral path for impressions of the muscular sense and tactile discrimination, and lesions in them cause ataxia.
They also lie at the dividing of the ways; the direct path con tinuing through the fillet decussation and the medial fillet to the thalamus, and the indirect path running through the arcu ate fibers to the cerebellar cortex.
Cortical Connection.—From the cerebellar cortex the impulses proceed cerebralward through Purkinje's neurones to the dentate nucleus and thence through the dentate neurones by way of the brachium conjunctivum cerebelli to the opposite red nucleus and thalamus. The cortical fillet conducts all common sensory impulses from the thalamus to the cerebral cortex.
Reflex Connections.—En route through the cerebellar cortex, coordinating reflex impulses are excited which proceed through cortical axones of the Purkinje cells to the nuclei of the cerebel lum; and then from those nuclei through the cerebello-teg mental axones in the restiform body and brachium conjunc tivum, first, directly to motor nuclei of the brain-stem and second, through the intermediation of the rubro-spinal, thalamo spinal and vestibulo-spinal tracts, to the motor nuclei of all cranial and spinal nerves.
The nucleus olivaris inferior, the olivary nucleus of the medulla (Figs. 123 and 125) is a sinuous, pouch-like collection of gray matter resembling the nucleus dentatus of the cerebellum. It is situated near the lateral surface of the medulla and is invested superficially and deeply by fibers from the lateral fasciculus proprius. Its open hilus looks medially and is filled with fibers, the olivo-cerebellar fibers, which join it to the opposite hemisphere of the cerebellum. On either side of the olivary nucleus is an accessory nucleus—the medial accessory, in the anterior column among the fibers of the interolivary part of the medial fillet, and the dorsal accessory in the lateral column. The olivary nucleus, covered by fibers of the lateral fasciculus proprius, forms the olive (oliva). The olive shows the longi tudinal extent of the nucleus and on section it is seen to measure 6 mm. (0.25 inch) in depth. The olivary nucleus is said to be a modern structure; it is found well developed only in the higher mammals. It is a relay station between the cerebrum and the cerebellum and between the spinal cord and the cerebellum. It receives the thalamo-olivary fasciculus from the cerebrum and the spino-olivary fasciculus (triangular tract of Helwig) from the spinal cord; axones of the nucleus gracilis and nucleus cuneatus also terminate in it. The cell-bodies of the nucleus are small; they give off very rich dendritic processes from all sides, which are closely massed about the cell-bodies, and a single, slender axone from each cell-body. The axones issue largely from the hilus of the nucleus as olivary peduncle; others emerge from the medial lamina of the olivary nucleus; nearly all the axones cross the median raphe and, piercing the opposite nucleus, continue as olivo-cerebellar fasciculus to the cortex of the cerebellar hemisphere and worm. According to Holmes and Stewart the olivo-cerebellar fibers have a definite arrangement; those from the dorsal fold end in the cortex of the superior sur face, while those from the ventral fold terminate in the inferior surface; fibers from the lateral part of the nucleus end in the lateral part of the hemisphere, and fibers from the region of the hilus and from the medial accessory nucleus terminate within the worm and the medial portion of the hemisphere. By far the larger number of fibers decussate to the opposite side (Brain, Vol. 31).
The neurones of the inferior olivary nuclei are emigrants from the rhom bic lip. The cells reach their positions through the olivo-arcuate migra tion (Essick), which begins early in the second month (in a zo mm. foetus) and continues into the third month (143 mm. foetus). The nucleus is well outlined at the end of the second month, though it possesses but a small part of its full quota of cells. At first the migration is intramedullary only, later it is in part superficial. The cell-bodies cross the median plane, impelled by the growing axones; with the development of the cerebellum, the axones extend through the restiform body to the hemispheres and vermis cerebelli; these, with other fibers entering the cerebellum, bear along with them from the rhombic lip the stellate and granular cells of the cerebellar cortex.
Lesions in the medulla are very fatal and death usually occurs before any sensory or motor phenomena can be observed; but rarely the pyramidal tracts alone have been involved or the pyramidal tracts together with one or more of the roots of the ninth to the twelfth cerebral nerves. In the last case, crossed paralysis is produced, as in the pons, affecting the cerebral nerves on the same side and the opposite spinal nerves. In progressive bulbar paralysis the motor nuclei of the medulla are involved as a preliminary to the degeneration of the anterior gray columna in the spinal cord.