NUCLEAR OR GANGLIONAR GRAY MATTER The nuclei of the cerebellum are the nucleus dentatus, nucleus emboliformis, nucleus globosus and nucleus fastigii (Figs. 107 and io). All these nuclei are made up of stellate cell-bodies, which vary in size from 20-8o microns. They form relay stations in the paths going out of the cerebellum. They originate all cerebello-fugal fibers, called cerebello-tegmental fasciculi. In them terminate axones of Purkinje's cells.
The nucleus dentatus (corpus dentatum) is a wavy, sinuous pouch of yellowish-brown gray matter imbedded in the medul lary body of each hemisphere. The nucleus dentatus meas sure 15-20 mm. in length and 7-10 mm. in width (Fig. 107). It is filled with white fibers, which issue from its open anterior end, called the hilus, and form the greater part of the brachium conjunctivum cerebelli. It also receives many axones from Purkinje's cells and, thus, forms a relay in the common sen sory path, also in the cerebello-rubro-spinal coordinating path.
The small nuclei are visible to the naked eye under favor able conditions (Fig. ro). One of these, a club-shaped mass, the cork-like nucleus emboliformis, partly closes the hilus of the dentate nucleus. It measures 15 mm. in length, 6 mm. in width and 3 mm. in thickness. It is in part continuous with the dentate nucleus and is closely allied to it in function, re ceiving cortical axones and contributing its own to the brachium conjunctivum. Medial to it is an elongated antero-posterior nucleus, bulbous in front, called the nucleus globosus. The nucleus globosus is intimately related to the emboliform nucleus; and, like it, is a dissociated part of the nucleus dentatus. These three nuclei probably represent the lateral cerebellar nucleus found in lower vertebrates (Edinger). The spherical head of the nucleus globosus, somewhat flattened on either side, lies just above the tonsil; it measures 5 mm. in diameter: its slender tail extends backward about 8 mm. (Piersol). The third small nucleus of the cerebellum lies next the median plane, in the anterior end of the vermis. It is just above the fastigium of the fourth ventricle and is called the nucleus of the highest point of the roof, nucleus fastigii (Stillingi). The nucleus fastigii is not found in the lower vertebrates; it first appears in the turtle (Chelone midas) and is well developed only in birds and mammals (Edinger). In man it is of ovoid shape, ro mm. long, circular in cross section and 5 mm. in diameter. It lies between the anterior and posterior com missures of the cerebellum, and is joined to its mate by the fa stigial commi s sure.
The nucleus fastigii contains very large stellate cells, 40-80 microns in diameter; the cells of the nuclei globosus, emboli formis and dentatus are stellate in form, but measure only 20 30 microns. All nuclear neurones of the cerebellum receive
axones of Purkinje's cells in the cortex. The axones of the small cells pass chiefly through the brachium conjunctivum to red nucleus and thalamus, and, according to Cajal, give off collaterals to the motor nuclei of mid-brain, pons and medulla. These form the superior group of cerebello-tegmental fibers. They form a link in a coordination path especially concerned with locomotion (Horsley). The large cells of the nucleus fastigii receive, in addition to the cortical axones, vestibular fibers both directly from the vestibular nerve and from the vestibular nuclei. The greater number decussate in the ver mis before entering the nucleus. The axones of the fastigial neurones decussate in the same situation and descend with some axones of the small-celled nuclei to the nucleus of Deiters in the medulla and to the motor nuclei of certain cranial nerves (V, VII, X); they form the fastigio-bulbar fasciculus of the cerebello-tegmental fibers. The nucleus fastigii is a part of the vestibular mechanism of equilibrium.
The White Substance of the Cerebellum (Figs. 104 and 107). —The corpus medullare contains all the white matter of the cerebellum. It is a strong body measuring 9 mm. OA in.) in thickness vertically in the middle of the hemisphere, but in the worm it is a thin sheet and is very slender as seen in a median section. Its branches to the cerebellar gyri are called the medullary laminae (lamince medullares). Viewed in a sagittal section of the hemisphere, the medullary lamina are short and stubby branches of a very thick trunk; but the tree-like ap pearance of the medullary body and laminae in the vermis is perfect, hence the name, arbor vitce, which is applied to them there. In the anterior cerebellar notch the medullary body divides into a thick superior lamina and a thin inferior lamina which are separated by a transverse furrow, the bottom of which constitutes the peak, or fastigium, of the fourth ventricle. The inferior lamina is the inferior medullary velum, already described; this, with the continuation of its ependymal epi thelium, forms the roof of the inferior half of the fourth ventricle. The superior lamina of the corpus medullare forms the three pairs of connecting bands (peduncles) and the superior medullary velum. Medullated axones make up the entire corpus medullare and its divisions. We study these axones in three systems like those of the cerebrum: I. Projection, or peduncular fibers.
II. Commissural fibers.
III. Association fibers.