Abstract
The caudal end of the human facial nucleus is on the level of the cranial part of the inferior olive and its cranial end somewhat on a higher level than the lower part of the abducens nucleus. The total length of the nucleus ranges between 4.4 and 4.8mm, a value somewhat larger than what has been reported to date. This discrepancy is perhaps due to the fact that I have taken into account the considerable extension of the caudal and the cranial end parts where the nerve cells forming the nucleus abruptly decrease in number. The length of the main part of the nucleus where the nerve cells are numerous and close-packed is about just 4mm, so that the end parts with decreased cells total 0.4-0.8mm in length. In transverse sections, the nucleus in its median part with densely packed nerve cells measures 2.5-2.7mm in the major and 1.5-1.7mm in the minor diameter.
The nerve cells in the facial nucleus show a rather perceptible group arrangement, but in the case of human facial nucleus, the opinions of the past researchers have been rather a variance on this grouping, for the cell groups are not marked off by fibre capsules, as in the case of cat, and they show varied fusion and partition on different levels. The cranial part of the human facial nucleus is represented as a small round patch of nerve cells on cross sections. About 0.3mm downwards of the cranial end, the tripartite basal form of the dorsal, the intermediate and the ventral groups begins to appear. Toward the caudal part, the nerve cells speedily gain in number, and in some places, the intermediate group is parted into the medial and the lateral subgroups and the ventral group into the lateral, the middle and the medial subgroups. Upon approaching the caudal end, first the dorsal group fades out, then the intermediate group, the caudal end part consisting of an extension of the ventral group alone. The nerve cells also gradually decrease in the end part finally to nothing. The caudal end part amounts to ca. 0.4-0.5mm in length.
As the human facial nucleus is not covered by fine nerve bundles, its boundary is not sharply defined. Consequently, some motor nerve cells wander out into the neighboring substantia gelatinosa, trapezoid body, formatio reticularis etc.
The motor cells in the facial nucleus are rather densely arranged, are 40-60μ in diameter, have several nerve processes each, show concave outline in the parts between the processes and give the virils appearance common to the motor cells in the other motor nuclei. The nerve processes are always smooth-surfaced. The axis cylinders are finer but darker-staining than the short processes and number one per cell usually, but sometimes we see 2 of them emerging form one cell and sometimes a single axis cylinder is found bifurcating. The short processes are filled with weak-staining neurofibrils, are very stoutly formed and are branched out into several rami. Their terminal fibres gradually taper off into sharp points.
The motor nerve cells are usually uninuclear, but some of them are binuclear. The cell nuclei are located centrally in the cell bodies, are round and contain chromatin granules which are very weakstaining, unlike those in vegetative cells, but the nucleoli darkstaining. The content of yellow pigment granules in the motor cells is particularlarly large. The granules are aggregated locally in the cell bodies, but nearly no neurofibrils were found running through these pigment aggregates.
Other nerve cells than motor cells are contained in the human facial nucleus as in the feline facialis (MORITA). The small-sized vegetative nerve cells are less numerous than in the cat facial nucleus and are found in particular frequency in the dorsomedial part of the nucleus. Most of them are spindle-form and contain mostly eccentrically standing nuclei provided with dark-stained chromatin granules, unlike those in the motor cells.
