296 AN AMERICAN TEXT- BOOK OF PHYSIOLOGY. 



energy increasing toward maturity. During middle life the two processes 

 are more nearly in equilibrium, though the total expenditure of energy is 

 probably greatest then ; and finally in old age the total expenditure of energy 

 diminishes, while at the same time the anabolic processes become less and 

 less competent to repair the waste. The question why in the nervous system 

 the energies wane with advanced age is but the obverse of the question why 

 they wax during the growing period. The essential nature of these changes 

 is in both instances equally obscure. 



Decrease in Weight of the Brain. — Between the fiftieth and sixtieth years 

 of life there is a decrease in the bulk of the encephalon in those persons 

 belonging to the classes from which the greater number of the records have 

 been obtained. So far as can be seen from the present records, there is no 

 marked change in the proportional development of the encephalon in old 

 age, though the loss appears to be slightly greater in the cerebral hemi- 

 spheres than in the other portions. 



Changes in the Encephalon. — 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 observations of Vulpius, show a senile decrease in 

 the number of fibres composing them. This decrease is more marked in the 

 motor than in the sensory areas. The time at which it commences cannot, 

 however, be accurately stated, owing to the small number of records after the 

 thirty-third year. Where records have been 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 year the decrease is 

 clearly shown. Engel has shown that the branches of the arbor vita 3 of 

 the human cerebellum decrease in size and number in old age. 1 



Changes in the Cerebellum. — Tn the case of a man dying of old age 

 (Hodge) some cells in the cerebellum were found shrunken and others (cells 

 of Purkinje) had completely disappeared. In the antennary ganglion of 

 bees a very striking difference appears between those dying of old age and 

 the adult just emerged from its larval skin. These changes are comparable 

 with those described in mammals, and it further appears that in passing from 

 the youngesl to the oldest forms cells have disappeared from the ganglia 

 and that in the young form of the bee there are some twenty-nine cells 

 presenl for each one found at a later period. 



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 arc atrophied and degenerated ; the supporting tissues increase, 

 and the walls of the small blood-vessels are thickened. These changes have 

 been found principally in the spinal cord, being most marked in the lumbar 



' Engel: Wiener medicinisehe Woehensehrift, 18f>3. 



2 Ketch. -r : Zeitechrififur Heilkunde, 1892; Redlich: Jahrbueh fur Psychiatric, 1893. 



