104 INTRODUCTION TO NEUROLOGY 



contact with the axons of other neurons with which they are in 

 synaptic union, thus increasing the resistance to nerve conduc- 

 tion at the synapse. 



Many physiological experiments show that, though the predis- 

 position to sleep may be brought about by the accumulation of 

 toxins in the blood or by other general causes, the actual falling 

 asleep is accompanied by a fall in blood-pressure, which may be 

 the essential factor in sleep. Fatigue of the vasomotor center 

 has been suggested as the real physiological cause of sleep. No 

 adequate proof of any of these theories has been brought for- 

 ward. 



The numerous theories regarding the neurological processes 

 taking place in the cerebral cortex during the progress of such 

 mental functions as attention, association of ideas, etc., 

 are likewise as yet entirely unproved. It has been suggested 

 that during cerebral function the resistance of some pathways 

 may be diminished by the ameboid outgrowth of the dendrites 

 so as to effect more intimate synaptic union with the physiolog- 

 ically related neurons, while the resistance of other paths may 

 be increased by the retraction of dendrites from their synapses. 

 Others believe that the neuroglia may participate in the process 

 by thrusting out ameboid processes between the nervous ter- 

 minals in the synapses and thus increasing the resistance. 

 Lugaro has suggested a different interpretation, in accordance 

 with which during sleep there is a generally diffused extension 

 of all nervous processes, thus providing for the uniform diffusion 

 of incoming stimuli, while in the state of attention all of these 

 processes retract save those which are directed in some definite 

 direction, thus narrowing the stream of nervous discharge so as to 

 intensify it and direct it into the appropriate centers. There is 

 no direct evidence for any of these theories, and the scientific- 

 ally correct attitude toward them is frankly to admit that at 

 present we do not know what physiological processes are in- 

 volved in any of these functions. 



Summary. The forms assumed by neurons are shaped in 

 part by their nutritive requirements and in part by their func- 

 tional connections. The metabolism of nervous protoplasm, as 

 measured by its COz output, is found to be as active in nerve- 

 fibers as in the cell bodies. In a nerve-fiber the metabolic 



