HISTOLOGICAL ELEMENTS 



825 



way the nervous impulse may be switched like a railway-train from 

 one path to another. But there is no experimental basis for this some- 

 what crude, if fascinating, hypothesis. Sherrington has suggested 

 that the presence of a ' membrane ' at the synapse may limit the con- 

 duction and determine its direction. Some membranes, such as frog's 

 skin, are known to possess a so-called irreciprocal permeability for 

 certain substances, permitting them to pass more easily in one direc- 

 tion than the other, and it is conceivable that a membrane at the 

 synapse might have a similar action in respect to the movement of ions 

 concerned in the propagation of the nervous excitation. Whatever 



Fig. 321. Nerve-Cells of Hirudo (Schafer. 

 after Apathy). A, unipolar motor cell; 

 a, network of neuro-fibrils near the sur- 

 face of the cell; b, near the nucleus n ; 

 c, afferent, d, efferent neuro-fibril. B, bi- 

 polar sensory cell a with its nucleus n ; 

 cu, cuticle; ep, epidermis cells between 

 which a neuro-fibril passes up from its 

 branched ending near the surface of the 

 skin to the nerve-cell, where it forms a 

 network, which gives off a fibril passing 

 towards the central nervous system. 



Fig. 322. Large Pyramidal Cell of 

 Cerebral Cortex (Barker, after Bech- 

 terew). a, axon; b, dendrite. 



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 electrical response (p. 797) 

 in the anterior root of the same segment, while no electrical change is 

 produced in the posterior root by stimulation of the anterior. We shall 

 see later on (p. 844) that some of the fibres of the posterior root and 

 their collaterals end by arborizing around the dendrites of the cells ol 

 the anterior horn. The excitation is, therefore, able to pass from the 



