248 HUMAN BIOLOGY 



lasting about 3^:500 of a second, and travelling about 

 88 yards a second along the conducting paths. Each 

 impulse leaves the conductor behind it in a state incapable 

 of transmitting another impulse for about the same period. 



Similarly the messages issuing from a nervous center 

 whatever they have to effect, whether to cause a gland to 

 secrete, a muscle to contract, or the heart to slacken, consist 

 solely of nerve impulses hke those generated by the receptors 

 at the beginnings of their entrant paths. Some of the nerve 

 fibers are thicker than others, and in these their impulses 

 travel sHghtly faster than those of the smaller. Otherwise 

 the only difference observed between impulses, wherever 

 occurring in the nervous system, hes not in the individual 

 impulse itself but in the time-grouping of the impulses, so 

 that concomitant with intensity of actions go impulse trains 

 of higher frequency (although each impulse in each train is still 

 quite discrete) , so that the number of successive impulses arriv- 

 ing or leaving by a particular pa1^h is per second greater. 



But impulses are not the sole form of functional reaction 

 exhibited by the nervous system. Consisting as it does of 

 chains of conducting cells laid end to end, the impulse 

 after propagating itself along one cell has then to excite 

 the next. All points of hnkage between cell and cell in the 

 system are confined to nerve centers. It is in nerve centers 

 that functional study finds evidence of forms of reaction 

 which summate, that is can add themselves together both 

 in space and in time, which nervous impulses cannot do. 

 These reactions which can show summation do so chiefly 

 in regard to the excitation of one cell by the next cell or 

 cells down stream from or collateral to it in the cell chain 

 as followed in its functional direction. Further at these 

 neurone junctions (synapses), which always lie within the 

 nervous centers, a process which is the polar opposite of 

 excitation is found, i.e., inhibition. It, like excitation just 

 mentioned, gives evidence of summation, but has for result 

 the prevention or diminution or suppression of excitation. 

 There is, however, no evidence that it can suppress impulses 

 once started to traverse the nerve fiber. These processes 

 of excitation and inhibition can much exceed in duration 

 the brief nervous impulse itself. 



