The terminal nucleus of the olfactory nerve is situated in the olfactory bulb (Figs. 31 and 86). This nucleus is connected with the cortical center in the hippocampal formation and with the amygdalate and habenular nuclei by the olfactory tract and striw-lateral, intermediate and medial (see p. 76). Reflex Connection.—In man there is the remnant of a very com plicated mechanism of olfactory reflexes. Some of its principal tracts are the following: the hippocampo-mammillary fasciculus, the fasciculus mammillaris princeps, the mammillo-thalamic and thalamo-spinal fasciculi; the mammillo-tegmental fas ciculus, the mammillary peduncle, and the dorsal longitudinal bundle of Schutz to genetic nuclei; the olfacto- and hippocampo habenular fasciculi, the habenulo-peduncular fasciculus, the interpedunculo-tegmental fasciculus, and both the dorsal longitudinal bundle of Schutz and the reticulo-spinal fas ciculus; the olfacto-mesencephalic fasciculus (Wallenberg) bears fibers directly from the cortex of the olfactory tract to the tegmentum of the mid-brain, pons and medulla and on into the spinal cord, supposedly to genetic nuclei. Cerebellar Connection. —The connection of the olfactory nerve with the cerebellum is problematic. Fibers of Wallenberg's bundle may reach the cerebellum; it is known that certain fibers of the stria medullaris thalami go beyond the nucleus habenuhu to the tectum of the mid-brain, especially to the superior colliculus; and nucleo cerebellar fibers enter the cerebellum as tecto-cerebellar tract, through the superior medullary velum. Thus olfactory nuclei are connected with the cerebellar cortex and coordinating arcs are completed as heretofore described (see olfactory projection and association neurones).
Genetic Nuclei (Nn. Origines) (Fig. r33).—The nuclei of the oculomotor, trochlear, abducent, facial, accessory and hypo glossal nerves and the nuclei of the motor roots of the tri geminal, glossopharyngeal and vagus nerves represent the an terior columna of gray matter in the cord and constitute the anterior columna series. The anterior columna and center of the gray crescent in the spinal cord contain two functional columns of efferent neurones, somatic and visceral, which are the counterpart of the two columns of afferent neurones in the pos terior columna. Though the gray crescent is broken up in the medulla by decussating tracts and the expansion of the ven tricle, all four columns are represented. The two afferent columns are described above; they form the somatic and visceral terminal nuclei, which receive the axones of peripheral afferent nerves. Genetic nuclei originate all efferent nerve fibers; and, since they supply three kinds of muscle (besides glands), there are three kinds of genetic nuclei, one somatic and two visceral (Malone): (I) The somatic nuclei of origin contain large cell bodies with massive, tigrous cytoplasm; they supply the striated voluntary muscles of the body (soma- body). (2) The common
visceral nuclei of origin are made up of cell-bodies which are nearly all nuclei; the cytoplasm is so scanty that the nuclei are relatively very large and the narrow rim of cytoplasm around the nucleus presents few tigroid bodies. These nuclei, through sympathetic ganglia, innervate the smooth involun tary muscles of viscera (and gland-cells). (3) The cardiac visceral nuclei of origin, through sympathetic ganglia, innervate the striated involuntary muscle of the heart. The cell-bodies of these nuclei are intermediate in size and in number of tigroid bodies between the somatic and the common visceral cells; the rim of cytoplasm is broader and more tigrous than in the com mon visceral neurone but not nearly equal in these particulars to the somatic neurones (Fig. 134) (Am. Jour. Anat., Vol. is).
The cortical connection of the visceral genetic nuclei has not been determined. Such connection is certainly present, but the tracts establishing it have not been traced. It is otherwise with the somatic nuclei of origin. These nuclei are connected with the cerebral cortex on both sides, but chiefly with that of the opposite hemisphere. The connection is established first and principally by the pyramidal tracts, some of the fibers running directly from the tract to the nucleus and others, leaving the tract high up, run through the accessory fillet (Bechterewi) to a point near the respective nuclei which they are about to enter; and, second, the fronto-pontal, temporo-pontal and intermediate paths are believed to send some fibers to the genetic nuclei of the same side. The simple reflex connection of these genetic nuclei is established for all of them by the medial longitudinal bundle; by the anterior tecto-spinal bundle and by the olivary pedicle (for the third, fourth, and sixth), by the trapezoid body (for the seventh) and by the spinal tract of the trigeminal nerve (for the fifth, seventh, and twelfth). The coordinating reflex mechanisms, of which the neurones of genetic nuclei form a part, are made up (I) of the afferent paths to the cerebellar cortex, and (2) of intermediate links between cerebellar cortex and the genetic nuclei. (1) The first is composed of afferent nerve-fibers which go directly to the cerebellum, and of afferent nerves and nucleo-cerebellar fibers, which rise in terminal nuclei and end in the cerebellar cortex. (2) The intermediate links are two or three in number, viz., the Purkinje neurones to cere bellar nuclei and the nuclear neurones which comprise the cerebello-tegmental tracts and terminate partly in genetic nuclei; and, again, these two sets of neurones together with one or more of the following tracts—the thalamo-spinal, the rubro spinal, the vestibulo-spinal, and the medial longitudinal tracts.