THE CENTRAL NERVOUS SYSTEM 



may limit the conduction and determine its direction. Some mem- 

 branes, such as frog's skin, are known to possess a so-called irre- 

 ciprocal permeability for certain substances, permitting them to 

 pass more easily in one direction than the other, and it is con- 

 ceivable that a membrane at the synapse might have a similar 

 action in respect to the movement of ions concerned in the propaga- 

 tion of the nervous excitation. Whatever the nature of the relation 

 between two superposed neurons may be, it does not permit the 

 conduction of nerve-impulses indiscriminately in both directions. 

 For instance, stimulation of the central end of the posterior root of 

 a spinal nerve causes an elec- 

 trical response (p. 719) in the 

 anterior root of the same seg- 

 ment, while no electrical 

 change is produced in the 

 posterior root by stimulation 

 of the anterior. We shall 

 see later on (p. 770) that 

 some of the fibres of the pos- 

 terior root and their collate- 

 rals end by arborizing around 

 the dendrites of the cells of 

 the anterior horn. The ex- 

 citation is, therefore, able to 

 pass from the telodendrions 

 of the posterior root-fibres 

 through the dendrites of the 

 anterior horn cells towards 

 their cell-bodies, but not in 

 the opposite direction, and 

 in general the direction of 

 conduction is from the den- 

 drites towards the cell-body. 

 Some investigators believe 

 that the fibrils already 

 spoken of as forming a felt- 

 work in the protoplasm of 

 the nerve-cell may run right 

 through from one cell to 

 another, thus constituting an 

 actual anatomical connec- 

 tion between the neurons, 

 and that such a connection 

 may be established also by 

 fibrils which do not enter 

 the cells at all, but run in 

 the intercellular substance of the grey matter. Such a continuity 

 of fibrils from cell to cell has been demonstrated in some of the 

 invertebrates e.g., in annelids (Fig. 303) where previously the 

 best examples of strictly isolated neurons were supposed to be 

 found (Apathy). The supporters of the theory of continuity look 

 upon the cell-body as merely necessary for the nutrition of the 

 nerve-net, but deny that it is necessary for the conduction of 

 nerve-impulses. If this is the case, it is obvious that the neurons 

 can no longer be considered as functional units in which the law of 

 isolated conduction of nerve-impulses (p. 689) holds good. Nor is 



FIG. 304. LARGE PYRAMIDAL CELL OF 

 CEREBRAL CORTEX (BARKER, AFTER BECH- 

 TEREW). 



a, axon ; b, dendrite. 



