FUNCTIONS OF THE CEREBRAL HEMISPHERES 459 



cord, the extra delay which occurs in the passage of the impulse through the 

 cord must take place either in the nerve cells themselves, or in the synapses, 

 through which the impulse passes from one neuron to the next in the chain of 

 reflex elements. 



The rate of passage of an impulse through the nerve cell can be deter- 

 mined only in one part of the body, viz. in the posterior spinal root ganglia, 

 since only in these is it possible to detect the moment of passage of an im- 

 pulse across a given section of a nerve fibre on both sides of the ganglion cell 

 in which the nerve fibres arise. Experiments on this point haye been made 

 by Steinach and by Moore. In each case the time occupied in the passage 

 of the impulse through the ganglion was not appreciably longer than if the 

 impulse had passed through a corresponding stretch of uninterrupted nerve 

 fibre. We are therefore justified in concluding that the relatively great 

 delay in the passage of an impulse through the central nervous system has 

 its seat in the synapses across which the impulse has to pass. This con- 

 clusion is in accordance with our experience on the latent period of muscle, 

 the greater part of which is due to changes occurring in the nerve endings, 

 i.e. in the synapses between motor nerve and muscle. The greater the 

 number of synapses involved in any given reaction, i.e. the greater the com- 

 plexity of the reaction, the longer will be the period which elapses between 

 the moment of application of the stimulus and the moment at which the 

 response takes place. Especially is this the case when the complex mesh- 

 work of neurons of the cerebral hemispheres is involved, or when the occur- 

 rence of the reaction is associated with the conscious processes of sensation 

 and volition. In the latter case the determination of the reaction time has 

 the added interest that it gives information as to the time relations of the 

 psychical processes which are the representation in consciousness of the 

 physiological changes occurring in the neurons of the central nervous 

 system. 



Many methods are employed for the measuring of the reaction time of conscious 

 processes. In most methods the application of the stimulus is arranged so as to close 

 the circuit of a current which flows through an electro-magnet activating a lever which 

 writes on a rapidly moving blackened surface. The reaction of the individual who 

 is the subject of experiment is arranged so that the resulting movement activates a 

 key by which the same current is opened. We thus obtain a tracing on the blackened 

 surface showing the moment of application of the stimulus and the moment at which the 

 reaction takes place. Thus, if the reaction time for an auditory stimulus is to be 

 determined, the electric current is arranged so as to pass through : 



(1) A spring contact key which can be pressed so as to make a noise. 



(2) An electric signal writing on a rapidly moving surface. 



'(3) A second key which the subject will release as soon as he hears the noise of 

 the first key and so break the current. 



The recording surface may be a drum, a pendulum myograph, or a spring myograph, 

 such as the ' shooter ' of du Bois-Reymond. If the sensory impression is to be from 

 the skin, the current may be made to pass through the primary coil of an inductorium 

 and wires be taken from the second coil to some part of the surface of the skin. In 

 this case the signal may be started by opening the circuit, and the subject of the experi- 

 ment will respond by closing the circuit by means of a spring key directly be feels 



