742 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



It is seen from this that the period of deep slumber is short, less than two 

 hours, and is followed by a long period, that of an average night's rest, during 

 which a comparatively slight stimulus is sufficient to awaken. Almost the 

 same results have been more recently obtained by Monninghoff and Pies- 

 bergen. 1 



It is evident that the effectiveness of such a stimulus is, however, no measure 

 of the recuperative processes in the central system. Repair is by no means 

 accomplished during the interval of deep sleep, and experience has shown, as 

 in the case of persons undertaking to walk a thousand miles in one thousand 

 hours, that although such an arrangement left the subject with two-thirds of 

 the total time for rest and refreshment, yet the feat was most difficult to accom- 

 plish by reason of the discontinuity in the sleep. The changes leading to 

 recuperation needed longer periods than those permitted by the conditions of 

 the experiment. 



Loss of Sleep. Loss of sleep is more damaging to the organism as a 

 whole than is starvation. It has been found (Manaceine) that in young dogs 

 which can recover from starvation extending over twenty days, loss of sleep for 

 five days or more was fatal. Toward the end of such a period the body-tem- 

 perature may fall as much as 8 C. below the normal and the reflexes disap- 

 pear. The red blood-corpuscles are first diminished in number, to be finally 

 increased during the last two days, when the animal refuses food. The most 

 widespread change in the tissues is a fatty degeneration, and in the nervous 

 system there were found capillary hemorrhages in the cerebral hemispheres, the 

 spinal cord appearing abnormally dry and anaemic. 



E. OLD AGE OF THE CENTRAL SYSTEM. 



Metabolism in the Nerve-cells. Connected closely with fatigue are those 

 alterations both of the constituent nerve-cells and of the entire system found in 

 old age. The picture of the changes in the living cells is that of anabolic and 

 katabolic processes always going on, but varying in their absolute and relative 

 intensity according to several conditions. Of these conditions one of the most 

 important is the age of the individual. In youth and during the growing 

 period of life the anabolic changes appear within the daily cycle of activity and 

 repose to overbalance the katabolic, the total expenditure of 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 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 Brain. The weight of the brain in advanced 

 life shows that between fifty and sixty years there is a decrease in the bulk of 

 the encephalon in those persons belonging to the classes from which the greater 

 1 Zeitschrijt Jiir Biologie, 1893, Bd. xix. 



