NERVUS GLOSSOPHARYNGEUS AND VAGUS.
1. Motor Portion. The efferent fibres arise partly within the nucleus motorius dorsalis nervi vagi el glossoftharyngei, which lies in the floor of the fourth ventricle lateral to the hypoglossal nucleus and medial to the nucleus alae cinereae ; the larger part of the fibres, however, arises from the cells of the nucleus ventralis or ambiguus, which lies within the formatio reticularis dorsal to the dorsal accessory olive. Since the voluntary innervation of the nucleus proceeds from the cerebral cortex, the path includes : a. The central the cerebral cortex to nucleus; b. The peripheral neurone—nucleus, peripheral nerve, muscle.
The root-bundles passing out from the dorsal nucleus are the equivalents of motor preganglionic sympathetic fibres destined for the innervation of involuntary muscle ; this nucleus, therefore, is also designated as the sympathetic motor nucleus. The fibres pro ceeding from the nucleus ambiguus, on the contrary, are for the voluntary muscle ; this nucleus, therefore, is known as the somatic motor nucleus. The latter consists of several groups of nerve-cells, the individual groups representing centres for the particular groups of muscles innervated by the vagus. The positions of these centres within the nucleus, however, are not yet sufficiently determined.
2. Sensory Portion. The efferent fibres arise within the ganglion superius el petrosum nervi glossopharyngei and ganglion jugulare et nodosum nervi vagi respectively. The peripherally directed branches form the peripheral sensory nerves ; the centrally directed branches enter the brain-stem as the sensory root-fibres and pass to the end nuclei. One part of the fibres ends within the nucleus alae cinereae, while another part forms a descending root, the tractus solitarius, and ends within the accompanying tract of gray substance, the nucleus tractus solitarii. The central neurones arise within the end-nuclei. The fibres emerging from the end-nuclei pass toward the mid-line and the interolivary layer, thence with the medial fillet to the thalamus. Within the latter, the third neurone takes origin and ends within the cerebral cortex. The sensory end-nuclei are also connected with the cerebellum, by means of the tractus nucleo-cerebellaris. Further, all of the centrally coursing sensory root-fibres do not terminate within the end-nuclei of the glossopharyngeus and vagus, since some of them join the descending tractus spinalis of the trigeminus nerve.