482 NERVOUS SYSTEM 



nerves as they are given off at different points and measuring the time 

 required for the appreciation of certain impressions and the beginning 

 of certain movements. While these observations are not absolutely 

 exact, their general results are of considerable physiological interest. 

 According to Burkhardt, the rate of motor conduction in the cord is 

 about one-third of the normal rate in the motor nerves As compared 

 with the sensory nerves, the cord conducts tactile impressions a little 

 faster and painful impressions less than one-half as fast. 



Attempts have been made to estimate the duration of acts involving 

 the central nervous system, such as the reflex phenomena of the spinal 

 cord or the operations of the cerebral hemispheres These have been 

 partially successful, or, at least, they have shown that the reflex and the 

 cerebral acts require a distinctly appreciable period of time. This in 

 itself is an important fact ; although the duration of these acts has not 

 been measured with absolute accuracy. As the general result of 

 experiments on these points, it has been found that the reflex action of 

 the spinal cord occupies more than twelve times the period required for 

 the transmission of stimulus or impressions through the nerves. Don- 

 ders found, in experiments on his own person, that an act of volition 

 required ^ of a second, and one of simple distinction or recognition of 

 an impression, ^ of a second. These estimates, however, are merely 

 approximate; and, until they attain greater accuracy, it is unnecessary 

 to describe in detail the apparatus employed. 



Aside from what have been described as reflex acts, it is found that 

 when impulses or impressions pass through cells, there is a distinct 

 retardation or delay in conduction, amounting to about 0.006 of a 

 second for afferent conduction in the spinal cord and about 0.05 for 

 efferent conduction. These figures, however, are approximations and 

 are interesting chiefly as indicating an action of nerve-cells different 

 from ordinary conduction by nerve-fibres, that has not been satisfac- 

 torily explained. It is probable that the delay is due to a redistribution 

 of the impulses by the cells to the synapses through which the conduc- 

 tion is continued in one direction or the other. 



Personal Equation. In recording astronomical observations, it 

 has been found that a certain time elapsed between the actual observa- 

 tion of a phenomenon and the moment of its record. This error, which 

 is equal to the interval of time between the impression made on the 

 retina and the muscular act by which a record is made, is not the same 

 in different persons or even in the same person at all times. It may 

 amount to \ of a second or even more, and it may be as low as -^ of a 

 second. If this difference is due to different rates of nervous conduc- 

 tion, and not entirely to variations in the rapidity of mental operations, 



