CENTRAL NERVOUS SYSTEM. 



187 



wall, and only later become the peculiar property of those portions which are 

 drawn out to form 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. Each dendrite may represent at least one pathway 

 by which impulses reach the cell-body. If, then, there are many dendrites, 

 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 

 dendrites, that the first branch to be formed is the axone, and that the com- 

 pletion of the full number of dendrites is a slow process. The pathways 

 formed by the dendrites are therefore continually increasing up to maturity 



Fig. 77.— Climbing fibre from human brain : a, nerve-fibre ; b, Purkinje's cell (Cajal). 



(Fig. 77). The relation between the " climbing fibre " and the dendrites of 

 the Purkinje cell illustrates this arrangement. 



Generation of Nerve-impulses. — The impulses which arrive at the cell- 

 body produce there chemical changes. These changes, when they reach a 

 given volume, cause a nerve-impulse which leaves the cell-body by way of 

 the axone. If the nerve-impulse is, as we assume, dependent on the chem- 

 ical changes occurring in the cytoplasm, then it must vary according to these 

 changes, which in turn can hardly be similar when the incoming impulses 

 that arouse them arrive along different dendrites. We know thai a stimulus 

 applied directly to the axone will give rise to a nerve-impulse; but, as we 

 shall see later, the chemical changes accompanying the passage of this 

 impulse are too slight to be detected. Whether in the cell-body equally 

 slight changes would give rise to an impulse cannot be determined. 



Birge 1 found upon stabbing the spinal cord of a frog with a needle in 

 1 Birge: Arch./. Anat. u. Physiol., Physiol. Abthl., 1882, S. 471. 



