THE DEVELOPMENT OF CERTAIN FEATURES OF THE CEREBELLUM. 



369 



in the former than in the latter. An average field in the vermis shows 36 cells, in 

 the flocculus 22, and in the hemisphere 45. 



In the infant 16 days old the Purkinje cells in these three structures have 

 maintained their relative number and size, being largest and least numerous in the 

 flocculus, smallest and most numerous in the hemispheres, and in the vermis occupy- 

 ing an intermediate position 

 as to size and number be- 

 tween the two extremes. An 

 average field in the flocculus 

 shows 19.6 cells (Zeiss Oc. 4, 

 Obj. A. A.), in the vermis 27, 

 and in the hemispheres 34. 

 Upon the theory that the ac- 

 tual number of Purkinje cells 

 in the cortex of a child of 16 

 days is the same as in a fetus of 7 months, we may regard the decrease in the num- 

 ber per field as inversely proportional to the increase in the growth of the cortex. 



In table A each number is the average of 20 different fields (Zeiss Oc. 4, Obj. 

 A. A.). A study of this table shows that although at 3 months the number of cells 

 per field is still greatest in the hemispheres, and greater in the vermis than in the 

 flocculus, the ratio is approaching 1: 1. 



TABLE A. 



FIG. 3. A contour drawing of the cerebellar hemisphere; a and b indicate 

 the position of the cell groups a and 6. The same difference noted 

 between cell group a, deeply placed, and cell group b, superficially 

 placed in the flocculus (fig. 2), are noted in this hemisphere also. 



If we may regard the number of cells per field as an index of the relative growth 

 of these different parts of the cerebellum, we are led to the conclusion that the cortex 

 of the cerebellar hemisphere increases 100 per cent during the first 3 months of 

 postnatal life, while the vermis undergoes an increase of 80 per cent, and the 

 flocculus an increase of 20 per cent. These percentages are indicative of the rela- 

 tively greater maturity of the flocculus at birth. At the age of 2 years the number 

 of cells per field is only a little below that of the brain of 3 months, representing a 

 uniform growth of about 30 per cent in the hemispheres, 20 per cent in the flocculus, 

 and 20 per cent in the vermis. It will be observed that the number of cells in the 

 three structures (flocculus 15, vermis 17.6, and hemisphere 16) corresponds almost 

 exactly to the respective number found in the adult (14.6, 16.9, and 15.6); from 

 which fact we may conclude that the cerebellum of a child of 2 years has nearly 

 reached its full development. 



Before taking up a discussion of table B let us note briefly the bearing of the 

 results above enumerated. The development of the Purkinje cells in the flocculus, 

 beginning early and progressing more rapidly than in the vermis, is very 



