202 FROM FISH TO PHILOSOPHER 



Most men spend a third of their Hves in sleep, a prac- 

 tically unconscious state, and yet the mechanism of sleep 

 is poorly understood. Consciousness waxes and wanes 

 like the aurora borealis as dreams come and go, shooting 

 its loosened streamers into the dark recesses of the for- 

 gotten or the half -forgotten, and leaving such faint traces 

 in memory that dreams are recalled with diflBculty when 

 one wakes up. Later we know that they are dreams, and 

 sometimes we know that they are dreams even while we 

 are dreaming, possibly because the flickering images are 

 unreal and out of focus. Some neurologists speak of a 

 'sleep center' that is presumed to inhibit cortical activity, 

 but with no greater cogency than others who speak 

 of a wakefulness center' that excites cortical activity. 

 Whatever the mechanism of sleep may be, it goes far 

 back in vertebrate history and seems to have its counter- 

 part even in some invertebrates. 



A blow on the head may cause loss of consciousness, 

 possibly by setting oflF a chaotic mass discharge of nerve 

 cells which itself has no pattern and momentarily dispels 

 all other patterns. This is possibly also the explanation 

 of loss of consciousness during the epileptic fit— a mas- 

 sive, unpattemed discharge of neural activity from some 

 hyperexcitable focus. Lack of oxygen causes uncon- 

 sciousness, because nerve cells cannot function without 

 oxygen, and fainting is attributable in most instances to 

 transient circulatory failure with a resulting drop in 

 blood pressure and a sharp decrease in blood flow and 

 oxygen supply to the brain. 



Events reach the level of consciousness only after they 

 have occurred, never simultaneously. Sensory nerve im- 

 pulses must travel from the periphery to the central 

 nervous system, and be distributed to one or more sen- 

 sory areas in the cerebral cortex, before they can enter 

 the field of consciousness. These impulses travel in the 

 human body at a rate of 3 to 30 meters per second (6 to 

 60 miles per hour) , and they are further delayed at every 

 synaptic junction by some three thousandths of a sec- 



