622 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Significance of Shape. Since the outgoing nerve-impulses pass along 

 the efferent cell-branches to their tips, it follows that if the impulses are 

 destined to leave the cell limits they will do so at the extremities of the 

 branches. This leads to the question how far the possession of branches is 

 necessary to the functional activity of a nerve-cell either for the reception or 

 transmission of an impulse. Since it has been pointed out that the spinal cord 

 of the newt and fish 1 is capable of conducting impulses even before the den- 

 drons of the cells composing it are developed, it follows that the transmission 

 of impulses is in some way dependent on the condition of the cell-wall inde- 

 pendent of cell-branches. This modification of the cell-wall may exist at 

 points where there are no branches, or during this early period be a general 

 property of the wall and only later become the peculiar property of that por- 

 tion which forms the tips of the branches. But not only the capacity to 

 receive, but also the capacity to deliver impulses is a function of the ends of 

 the branches, and the cell-wall at these points must therefore be peculiarly 

 modified with a still further differentiation determining the direction in which 

 the impulses may pass. If, therefore, the mature cell is thus arranged, its 

 shape and the number of its branches have a meaning. Each dendron repre- 

 sents at least one pathway by which impulses reach the cell-body. If, then, 

 there are many dendrons, the cell-body is subject to a more complicated series 

 of stimuli than if the branches are few. It will be remembered that the 

 young nerve-cell has no dendrous, that the first branch to be formed is the 

 neuron, and that the completion of the full number of dendrons is a slow 

 process. The pathways formed by the dendrons are, therefore, continually 

 increasing up to maturity. 



Effect of Impulses. The impulses which arrive at the cell-body produce 

 there chemical changes. These changes when they reach a given volume and 

 intensity cause a nerve-impulse which leaves the cell-body by way of the 

 neuron. If the nerve-impulse is, as we assume, dependent on the chemical 

 changes occurring in the cytoplasm, then the nerve-impulse must vary accord- 

 ing to these changes, which in turn can hardly be similar when the incoming 

 impulses that arouse them arrive along different dendrons. 



Concerning the modifications in the nerve-impulse as dependent on the 

 cell-body, there are thus far known only the variations in the intensity of the 

 negative variation, this being greater with the stronger stimulus. When the 

 nerve-impulses leave a cell-body after momentary stimulation, they depend not 

 upon a single event but a series of events, varying slightly for the different 

 groups of cells. Experiments showing the multiple character of the impulses 

 aroused within the central system have been made by Gotch and Horsley. 2 

 When the motor cortex of a monkey was stimulated (Fig. 153) by means of 

 the faradic current, the muscles which by this means were made to respond 

 showed a long tonic contraction followed by a series of shorter clonic ones 

 (Fig. 154, _D). When the spinal cord had been cut across, the cortex was 



1 His: Archiv filr Analomie und Physiologic, 1890. 



2 Proceedings of the Royal Society, London, 1888. 



