PROTECTIVE MECHANISMS OF THE NERVOUS SYSTEM 721 



by transudation into the subarachnoid and subdural spaces, and (c) by 

 transfer from the intra-adventitial lymphatic spaces of the cortex. 



Regarding its function it should be mentioned first of all that it 

 serves the general purpose of lymph, because it bathes the nerve-cells. 

 It forms a nutritive medium, which, in addition, is chemically protect- 

 ive, because it tends to exclude such harmful substances, as proteins 

 and toxins. In the second place, it is mechanically protective, be- 

 cause it serves as a water-bed upon which rests the base of the middle 

 and posterior parts of the encephalon, and which in other regions mini- 

 mizes the force of blows upon the integument. In the third place, it 

 may be conjectured that this fluid subjects the nervous tissue to a 

 certain pressure which keeps it under a tension best adapted for its 

 function. 



Sleep. 1 Activity must always be followed by rest, because con- 

 tinued catabolic processes lead to fatigue and exhaustion. This holds 

 true of the motor as well as of the sensory mechanism. A heart or 

 skeletal muscle, when made to contract repeatedly, soon loses its irri- 

 tability, because it is not allowed sufficient time to replenish the mate- 

 rial lost during its periods of activity. This is also true of glands if 

 made to secrete excessively, and of all sense-organs, if stimulated for a 

 long period of tune. What is true of individual tissues, is true of the 

 body as a whole. It requires a period of recuperation during which its 

 receptive power is at a minimum and during which it reestablishes 

 proper physico-chemical conditions. Sleep is a universal phenomenon 

 among animals; even the fish, reptiles and amphibians pass long 

 periods of time in absolute quietude. The fundamental reason for 

 sleep must, of course, be sought in chemico-physical changes, but our 

 present knowledge of metabolism is not sufficiently advanced to ex- 

 plain the cause of this phenomenon in more than a very general 

 manner. 



While the daily requirement of sleep is about 7-8 hours for the 

 adult, this time varies considerably under different conditions. In 

 childhood, the amount of stored energy is small, owing to the immature 

 development of the cells, while the metabolism, relatively rated, is 

 intense, and hence, children require longer periods for recuperation. 

 In old age, on the other hand, the small amount of stored energy is due 

 rather to retrogressive changes and is associated with a lower meta- 

 bolism, both factors combining to produce shorter hours of sleep. In 

 either case, however, loss of sleep is injurious and even more so than 

 the withholding of nourishment. Thus, Manaceine 2 has found that 

 young dogs may withstand a starvation period lasting twenty days, 

 but fail to recover from a loss of sleep extending over more than four or 

 five days. These animals exhibited a fall in the body temperature of as 

 much as 8 C. below normal, a diminution of the reflexes, fatty de- 

 generation of diverse tissues, and hemorrhagic extravasations into the 



1 Pie"ron, Le probleme physiol. du sommeil,_ Paris, 1913. 



2 Contemp. Science Series, London, 1897. 

 46 



