In size a granule of the cerebellum presents a marked contrast to a motor cell of the spinal cord; yet it is formed on essentially the same plan. From its minute round body (about 8 µ in diameter) four or five slender dendritic processes are drawn out. Each dendrite ends in a little bunch of twigs, resembling fingers curved over the palm. Its single slender axon runs towards the surface of the cortex. As the granules lie at a considerable depth, this course is, for those which distribute to the most superficial layers, a long one. They pass from the granular to the molecular layer between the big cells of Purkinje. When the axon has reached a certain level in the molecular layer, it divides into two threads which run for a great distance, right and left.
The granules of the cerebellum have a curious developmental history. Every neurone in the body has a lifelong existence. Except for the rare accident of its destruction by disease it occupies its station to the hour of death. But at the time of birth many neurones are still immature. Not all the granules of the cerebellum have yet assumed their permanent form or situation. Beneath the pia mater there is still a layer of minute undifferentiated cells. These, as they grow into granules, elongate, in the first instance, into long spindles. Subsequently they sink down through the molecular layer and between the cells of Purkinje, leaving the poles of the spindle as the right and left divisions of the axon (Fig. 21). It is interesting to learn that such a migration is possible. It is also of interest to find that a tiny granule of the cerebellum goes through the same stages in attaining its adult form as one of the large cells of a spinal ganglion.
[Illustration: FIG. 21.—THE GROWTH AND MIGRATION OF GRANULES OF THE CEREBELLUM.
Half a dozen nuclei of as yet undeveloped granules are seen lying beneath the pia mater. From this level to the bottom of the drawing granules are shown in successive stages of growth. These developing granules, selected from various preparations of the cortex of the cerebellum, were drawn from nature.]