THE NERVOUS SYSTEM 303 



by thorns with the body and dendrites of the nerve-cell Z, 

 then the net is the meeting-ground of all the routes through 

 which Z is called into action. A nerve-wave from any of the 

 neurones A to Y, breaking upon this net, passes along the 

 thorns into the protoplasm of Z. 



In size a granule of the cerebellum presents a marked con- 

 trast to a motor cell of the spinal cord ; yet it is formed on 

 essentially the same plan. From its minute round body 

 (about 8 p 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 dis- 

 tribute 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 undifferentiatei 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 cere- 

 bellum goes through the same stages in attaining its adult 

 form as one of the large cells of a spinal ganglion. 



There are many different types of neurone. Any attempt 

 to describe them, or to give an account of the various details 

 of structure which recent improvements in technique have 

 enabled anatomists to observe, would fill a lengthy treatise ; 

 and would, moreover, be beside our aim, which is limited to 

 obtaining such an idea of the unit of the nervous system as will 



