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THE GROWTH OF THE BRAIN. 



growth, the complex nervous system of the adult is 

 finally elaborated. At first the walls are formed by a 

 thin layer one or two cells in thickness, then there 

 appear next to the central canal, germinal cells which 

 rapidly divide. A portion of the cells thus formed 

 become young nerve cells neuroblasts another portion 

 build up the framework the neurospongium in which 

 the nerve elements lie. With these changes the cells 



FlG. 19. A group of human nerve cells drawn to scale, X 200 

 diameters. A, cell from ventral horn of spinal cord ; />, cell 

 from the spinal ganglion of dorsal root, with its nerve pro- 

 cess ; C, cell from the column of Clarke; I), "Solitary" cell 

 from the dorsal horn of the spinal cord ; E 9 cross-section of 

 a large nerve fibre ; /% granule cells from the cortex of the 

 cerebellum. (Modified from Waller, Human Physiology, 

 1893.) 



increase in number, then in size, and their form becomes 

 more complicated, while at the same time they separate 

 more widely from one another. To understand these 

 processes, it will be of advantage to describe one or two 

 of the commoner forms of nerve cells after they have 

 reached maturity, returning later to follow them from 

 their first to their final shape. Formerly the terms 

 employed were nerve cell and nerve fibre, with the 



