328 THE GROWTH OF THE BRAIN. 



The thickness of the cerebral cortex diminishes in 

 harmony with the shrinkage of the entire system. In 

 large measure this must depend on the loss of volume 

 in the various fibre systems, which, according to the 

 tables from Vulpius (Chapter XIV.), show a senile 

 decrease in the number of fibres composing them. This 

 decrease is more marked in the motor than in the sen- 

 sory areas. The time at which it commences cannot, 

 however, be well judged from the curve mentioned, 

 owing to the small number of records after the thirty- 

 third year. Where records are made between this and 

 the seventy-ninth year, it appears that there is no decided 

 diminution until after the fiftieth year, though at the 

 seventy-ninth the decrease is always clearly shown. 

 Engel has shown that the branches of the arbor vitae 

 of the human cerebellum decrease in size and number 

 in old age. See Fig. 45. 1 



To the anatomy of the human nervous system in old 

 age contributions have been made by studies on the 

 pathological anatomy of paralysis agitans. 2 



In subjects suffering from this affection the bodies of 

 the nerve cells are shrunken, pigmented, and show in 

 some cases a granular degeneration ; the fibres, in part, 

 are atrophied and degenerated, the supporting tissues 

 increased, and the walls of the small blood-vessels 

 thickened. These changes have been found principally 

 in the spinal cord, being most marked in the lumbar 

 region. But the cords of the aged persons who do not 

 exhibit the symptoms of paralysis agitans show similar 

 changes, though usually they are not so marked, and 

 hence the pathologic anatomy of this disease resolves 

 itself into a somewhat premature and excessive senility 

 of the central system. 



1 Engel, Wien. Med. Wochcnschrift, 1863. 



- Ketcher, Zeitschrift f. Heilkunde, 1892; Red\\ch,ya/ir&uc/i. j. 

 Psyc/i/'a/rie, 1893. 



