236 THE GROWTH OF THE BRAIN. 



lengthening of all the fibre systems that run parallel 

 with its surface. 



These conditions being complex, it follows that the 

 character of the fissuration may vary greatly in con- 

 sequence of a slight alteration in them, and nearly 

 the same gross appearance be the result of dissimilar 

 combinations among them. For instance, a much- 

 fissured surface may be the result either of an ample 



FlG. 45. Silhouettes of the white substance (arbor vitas) exposed 

 by a longitudinal median section of the human cerebellum. 

 A, at birth ; B, type in males and in vigorous and muscular 

 persons ; C, type in females and in persons weak and aged. 

 Natural size. (Engel.) 



expansion of the cortical layer, or a lack in the forma- 

 tion of the subcortical fibres. 



The fibres in the encephalon become only slowly medul- 

 lated, so that from birth to maturity there is an increase 

 of the portion medullated. Engel I has shown this in 

 the case of the cerebellum. When a section in the median 

 plane is made through the cerebellum there is exposed 

 a central mass of white matter, having many branches, 

 and surrounded by the grey cortex. This "arbor vitae," 

 as it has been called, from its resemblance to the foliage 

 of the tree bearing that name, changes in shape, and at 

 1 Engel, Wien. Med. Wochen., 1863. 



