CENTRAL NERVOUS SYSTEM. 619 



ever, that the impulses roused by artificial stimuli are usually accompanied by 

 a much stronger electrical variation than accompanies the normal impulses. 



In the peripheral system the nerve-impulse, when once started within a 

 fibre, is confined to that track and does not diffuse to other fibres running par- 

 allel with it in the same bundle. In other words, throughout this portion of 

 its course the conduction of the impulses is isolated. 



The above-mentioned facts have been observed on the peripheral nerves, 

 and these morphologically are but parts of the medullated neurons, the cell- 

 bodies of which are located either in the central system proper or in the spinal 

 or sympathetic ganglia. 



The observations apply therefore to but one portion of the nerve-cell, and 

 our present purpose is to determine how far it is possible to extend them so 

 that they apply to the entire nerve-cell, noting at the same time the modifica- 

 tions introduced by this extension. 



Conditions Surrounding- the Extension of the Nerve-impulse. 

 Owing to the small size of nerve-cell bodies, there are of course very few 

 instances in which a single nerve-cell, or part of such a cell, has been the 

 object of direct physiological experiment. 



Groups of elements are usually employed like those represented in the 

 groups of neurons forming the various peripheral nerves, and where these 

 have common functions, the inference may be made from the changes in the 

 mass to changes in the constituent units. This method can be used without 

 serious error, and it is possible, therefore, to speak of events occurring in the 

 individual elements, although the experiments were made upon masses of 

 them. 



Direction of the Nerve-impulse. In the case of a given nerve-cell, the 

 impulses which we usually consider pass in one direction only. For instance, 

 along the ventral nerve-roots of the spinal cord the impulses pass from the 

 cord to the periphery, while in the dorsal roots, so far as they take origin 

 from the cells of the spinal ganglia, these impulses travel in the opposite 

 direction. At the same time experiment has shown that if a nerve-trunk be 

 stimulated at a given point, then the nerve-impulse can be demonstrated as 

 passing away from the point of stimulation in both directions. 



We are therefore led to inquire what limits are set to the passage of im- 

 pulses in a direction opposite to the usual one. The narrowest limits, it 

 appears, are those of the single cell in which the impulse has originated. The 

 experimental observations are as follows : When the fibres forming the ven- 

 tral root of the spinal cord are stimulated electrically, and the cross section of 

 the cord, somewhat cephalad to the level at which the root joins it, is explored 

 with an electrometer, there is not found any evidence of nerve-impulses pass- 

 ing cephalad in the substance of the cord. The arrangement of the cells in 

 the cord is such, however, that the cell-bodies which give origin to the fibres 

 forming the ventral root are physiologically connected with fibres running 

 toward them from every portion of the cord, and under normal conditions 

 these fibres convey impulses to them. The experiment shows that when, under 



