CONDUCTION WITHIN NERVE CELLS. 609 



length in the various cases ; it is more than probable that they pass 

 through an increasing number of nerve units (i.e. nerve cells), according 

 to the increased complexity of the psychical process involved. And this, 

 according to the theory of isolated units (neurone theory), would mean 

 the passage across as many synapses, which are probably the parts of the 

 nerve chain where relative blocks occur. 



If we assume (and the assumption is not a very improbable one) 

 that in the same individual, under the same conditions, the lost time of 

 block at each synapse is about the same, we can arrive at a general 

 idea of the relative number of synapses which are passed ; in other 

 words, the number of nerve cells in a chain called into action in various 

 psychical processes. 



A careful study of the differences in reaction time, and of other examples 

 of " lost time," in the passage through nerve centres, may help us to form a 

 conception of the amount of delay which each synapse is responsible for. We 

 may therefore advantageously consider some of the facts which have been 

 accumulated upon this point. 



The reduced reflex time which occurs in the lower part of the frog's 

 spinal cord, when a reflex contraction of the homonymous gastrocnemius is 

 produced by stimulation of a posterior root, has been determined by Wundt, 1 

 who found the minimal amount to be only 0*008 second. 2 For a crossed 

 reflex the delay over and above this was only about 0*004 second, and the 

 same for a reflex movement of the other limb on the side stimulated. If we 

 assume that for the crossed reflex only one additional nerve cell is interpolated, 

 we obtain 0*004 second as the lost time of the synapse; whereas, if two 

 additional cells are assumed to be interpolated, the lost time at each synapse 

 would be only 0*002 second. Since the reflex has a lost time of 0*008 second, 

 at least two, and perhaps four, synapses are involved in the path of the most 

 simple reflex through the nerve centre. 



The reduced reflex time 3 of the closure of the eyelid on excitation of the 

 skin of the lid, was found by Exner to be as much as 0*047 second. 4 



It would appear probable, therefore, that the time occupied by any 

 simple reflex act is far too long to be produced merely by the passage 

 of the nervous impulses across a single set of synapses (e.g. between the 

 sensory and the motor cells), but we must assume a far greater com- 

 plexity of arrangement, and the intermediation of several intercalated 

 cells. 



The great difference between the time taken in the eyelid reflex and that 

 produced by stimulation of a posterior root, may be due to the fact that there 

 is a choice of paths for afferent impulses ; that under ordinary circumstances 

 of natural excitation of the nerve-endings in the skin, the nervous impulses take 

 a course tending to produce co-ordinate movements and involving a chain of 

 several nerve cells, but that in the abnormal conditions of direct and more severe 

 excitation of the nerve fibres, the impulses take the shortest possible course, 

 and call into play a less complex nervous mechanism. 



It was found by Exner 5 that the total lost time between the excitation of 



1 "Untersuch. z. Mechanik d. Nerven u. Nervencentren," Stuttgart, 1876, Abth. 2. 



2 Gotch found the reduced reflex for the electrical discharge in Malapterurus to be also 

 '008 second (see this volume, p. 589). 



3 I.e. the time obtained after deducting from the total time of the response (1) That 

 occupied in the passage of the sensory and motor nervous impulses from and to the eyelid ; 

 and (2) that occupied by the period of latent stimulation of the orbicularis palpebrarum. 



4 Arch.f. d. ges. PhysioL, Bonn, 1874, Bd. viii. S. 526. 



5 "Exper. Untersuch., etc.," Arch.f. d. ges. PhysioL, Bonn, Bd. viii. S. 532. 



VOL. u. 39 



