572 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



cms stimulation for several seconds, then another powerful contraction appears, 

 etc. The same thing has been observed by Lombard with continuous thermal 

 stimulation. 



These and similar facts teach us that when the nerve cell has discharged 

 an unusually strong impulse as the result of summation of its stimuli, it is 

 to a certain extent exhausted and requires a certain time to be recharged. 

 It is self-evident that the resistance of a cell to stimulation will depend upon 

 the mode, strength, and frequency of the stimulation, and we know from 

 everyday experience that nerve cells withstand the normal stimuli much 

 better than they do our relatively crude artificial stimuli. 



(3) Certain observations go to show that nerve cells, just like nerve 

 fibers and muscles (cf. page 429), have a refractory period. 



In stimulating the motor zone of the cerebral cortex Richet and A. Broca 

 observed that a second stimulus was ineffective if it follows the first at a shorter 

 interval than 0.1 second. The reflex closure of the eyelid to a second stimulus 

 does not take place if the second stimulus follows the first at a shorter interval 

 than 0.5-1 second (Zwaardemaker). According to Baglioni, the refractory 

 period of the sensory elements in the' spinal cord of a frog amounts to some 

 0.25-0.5 second. The inability of the normal spinal cord to mediate complete 

 tetanic contractions reflexly is to be explained by this circumstance. 



(4) Finally, artificial stimulation of nerve cells teaches us that they have 

 the ability to transmute the stimuli which they receive into a perfectly char- 

 acteristic rhythm. 



Stimulating the spinal cord of a rabbit with forty-three induction shocks 

 per second, Kronecker and Hall obtained muscular contractions showing a 

 rhythm of twenty per second, whereas on stimulating the peripheral nerve forty- 

 three times per second the contractions obtained had exactly the same rhythm 

 as the stimuli. We are not to suppose, however, that the frequency of the im- 

 pulses given off from the central nervous system is always the same. It appears 

 rather from the experiments of Stern on the muscular sound produced by stimu- 

 lation of different portions of the nervous system with induction shocks of dif- 

 ferent frequency, that the spinal cord is capable of discharging its impulses at 

 varying rhythms up to 230 per second, although a number of observations tend 

 to show (cf. page 431) that the frequency at which impulses are given off from 

 the central nervous system is in general very much lower than this. 



D. DEPENDENCE OF THE NERVE CELL UPON THE BLOOD SUPPLY AND 

 THE EFFECTS OF POISONOUS SUBSTANCES 



The nerve cells in the body are intensely active, hence they require an 

 abundant supply of blood. In fact, it has been observed that the large gan- 

 glion cells of the vagus and the trigeminal nerves in the bony fish, Lophius 

 piscatorius, have a small knot of capillaries of their own penetrating into their 

 substance and so supplying them with nourishment (Fritsch). 



When the blood supply to the brain is considerably reduced by compression 

 of the carotids on both sides, unconsciousness results, in many cases at least, 

 because the nerve cells are functionally incapacitated. 



