BLOCK BETWEEN NERVE CELLS. 613 



not by the same number but by a certain small multiple of it, has been found 

 to produce a further delay of about 0*075 second, and so on. 



Another question which may be here briefly considered, is, whether 

 the nervous impulses are actually passed on from one cell to the other 

 at the synapses, or whether new impulses are set up in the second cell, 

 as a result of a stimulation received from the first one. The only facts 

 which we have to guide us here are furnished by observations of the 

 following character : — One of the limb muscles of an animal (dog, 

 monkey) is connected with a myograph, and the part of the cerebral 

 cortex being first found, excitation of which produces contraction of the 

 muscle, the grey matter of that part of the cortex is removed, and 

 the electrodes are placed on the subjacent corona radiata. If now the 

 fibres of this, containing of course the fibres of the pyramidal tract 

 derived from the cells which have been removed, are stimulated by a 

 rapidly interrupted faradic current (50 per second), the muscle curve 

 shows an incomplete tetanus, with waves of about 10 or 12 per 

 second. 1 Now, since nervous impulses are being generated in the fibres 

 of the pyramidal tract at the rate of 50 per second, if they passed 

 unaltered to the cells from which the nerves to the muscle employed 

 take origin, the muscle should respond by a complete tetanus, as it 

 would do if the motor nerve itself were stimulated at this rate ; it is 

 therefore clear that a new rate of rhythm has been started in the 

 motor cells of the anterior horn, and it is probable that fresh nervous 

 impulses are set up within them. 2 In such a case as this, we may 

 conceive the stimulus which provokes the nerve impulses in the second 

 nerve cell to be the change of electrical potential (action-current) which 

 accompanies the nervous impulses in the fibres artificially stimulated. 

 But whatever the nature of the stimulus, if we assume that for each 

 nerve cell there is, as with other protoplasmic structures, a period of 

 latent excitation, the block which occurs at each synapse would thereby 

 be accounted for. 



It would appear that, while the synaptic mechanism produces only 

 a delay in the passage of nerve impulses which are travelling in the 

 direction which they normally take, it forms a complete block to passage 

 of impulses in the reverse direction (" valve " action). Thus when, after 

 the posterior roots are cut, the mixed nerve is stimulated, no effect what- 

 ever is produced upon the spinal cord. This is in marked contrast to 

 the effects got when the anterior roots are cut and the posterior left 

 intact ; for stimulation of the mixed nerve under these conditions pro- 

 duces, of course, very pronounced reflex effects. When, under the same 

 conditions (anterior roots cut, posterior intact), the spinal cord is stimu- 

 lated and the mixed nerve led off to the galvanometer, a decided effect 

 is obtained. 3 In the latter case, the course of the nerve impulses is 

 of course the reverse of normal, but since there is no synapse between 

 the peripheral sensory nerves and the fibres which pass up the posterior 



1 Horsley and Schafer, Journ. Physiol., Cambridge and London, vol. vii. 



2 The same thing is seen when a movement is produced refiexly by excitation of a 

 sensory nerve or of the skin. The excitation may be very rapid, or it may be continuous, 

 but it is responded to by the muscles with a relatively slow rhythm of contraction. This 

 rhythm must in all probability be set up in tbe motor nerve cell, unless we are to assume 

 that it is due to a tendency of the muscle itself to contract with this rhythm. It does not 

 do so if its nerve be directly stimulated at a rapid rate, although, if very excitable, it may 

 respond to a continuous electrical stimulation with a prolonged contraction which simu- 

 lates an irregular tetanus. 



3 Gotch and Horsley, Phil. Trans., London, 1891, vol. clxxxii. B, p. 267. 



